diff --git a/include/openthread/message.h b/include/openthread/message.h
index b1766ae7a..ed251baa5 100644
--- a/include/openthread/message.h
+++ b/include/openthread/message.h
@@ -321,11 +321,8 @@ void otMessageQueueInit(otMessageQueue *aQueue);
  * @param[in]  aQueue    A pointer to the message queue.
  * @param[in]  aMessage  The message to add.
  *
- * @retval OT_ERROR_NONE     Successfully added the message to the queue.
- * @retval OT_ERROR_ALREADY  The message is already enqueued in a queue.
- *
  */
-otError otMessageQueueEnqueue(otMessageQueue *aQueue, otMessage *aMessage);
+void otMessageQueueEnqueue(otMessageQueue *aQueue, otMessage *aMessage);
 
 /**
  * This function adds a message at the head/front of the given message queue.
@@ -333,11 +330,8 @@ otError otMessageQueueEnqueue(otMessageQueue *aQueue, otMessage *aMessage);
  * @param[in]  aQueue    A pointer to the message queue.
  * @param[in]  aMessage  The message to add.
  *
- * @retval OT_ERROR_NONE     Successfully added the message to the queue.
- * @retval OT_ERROR_ALREADY  The message is already enqueued in a queue.
- *
  */
-otError otMessageQueueEnqueueAtHead(otMessageQueue *aQueue, otMessage *aMessage);
+void otMessageQueueEnqueueAtHead(otMessageQueue *aQueue, otMessage *aMessage);
 
 /**
  * This function removes a message from the given message queue.
diff --git a/src/core/api/message_api.cpp b/src/core/api/message_api.cpp
index 922f413ae..406ee7016 100644
--- a/src/core/api/message_api.cpp
+++ b/src/core/api/message_api.cpp
@@ -118,18 +118,20 @@ void otMessageQueueInit(otMessageQueue *aQueue)
     aQueue->mData = NULL;
 }
 
-otError otMessageQueueEnqueue(otMessageQueue *aQueue, otMessage *aMessage)
+void otMessageQueueEnqueue(otMessageQueue *aQueue, otMessage *aMessage)
 {
     Message &     message = *static_cast<Message *>(aMessage);
     MessageQueue &queue   = *static_cast<MessageQueue *>(aQueue);
-    return queue.Enqueue(message);
+
+    queue.Enqueue(message);
 }
 
-otError otMessageQueueEnqueueAtHead(otMessageQueue *aQueue, otMessage *aMessage)
+void otMessageQueueEnqueueAtHead(otMessageQueue *aQueue, otMessage *aMessage)
 {
     Message &     message = *static_cast<Message *>(aMessage);
     MessageQueue &queue   = *static_cast<MessageQueue *>(aQueue);
-    return queue.Enqueue(message, MessageQueue::kQueuePositionHead);
+
+    queue.Enqueue(message, MessageQueue::kQueuePositionHead);
 }
 
 otError otMessageQueueDequeue(otMessageQueue *aQueue, otMessage *aMessage)
diff --git a/src/core/coap/coap.cpp b/src/core/coap/coap.cpp
index 0c4c394bf..af8259ce0 100644
--- a/src/core/coap/coap.cpp
+++ b/src/core/coap/coap.cpp
@@ -437,7 +437,7 @@ Message *CoapBase::CopyAndEnqueueMessage(const Message &aMessage, uint16_t aCopy
 
     mRetransmissionTimer.FireAtIfEarlier(aMetadata.mNextTimerShot);
 
-    IgnoreError(mPendingRequests.Enqueue(*messageCopy));
+    mPendingRequests.Enqueue(*messageCopy);
 
 exit:
 
@@ -848,7 +848,7 @@ void ResponsesQueue::EnqueueResponse(Message &               aMessage,
 
     VerifyOrExit(metadata.AppendTo(*responseCopy) == OT_ERROR_NONE, responseCopy->Free());
 
-    IgnoreError(mQueue.Enqueue(*responseCopy));
+    mQueue.Enqueue(*responseCopy);
 
     mTimer.FireAtIfEarlier(metadata.mDequeueTime);
 
diff --git a/src/core/coap/coap_message.hpp b/src/core/coap/coap_message.hpp
index 5683bb3d9..e75d4e3d3 100644
--- a/src/core/coap/coap_message.hpp
+++ b/src/core/coap/coap_message.hpp
@@ -678,11 +678,8 @@ public:
      *
      * @param[in]  aMessage  The message to add.
      *
-     * @retval OT_ERROR_NONE     Successfully added the message to the queue.
-     * @retval OT_ERROR_ALREADY  The message is already enqueued in a queue.
-     *
      */
-    otError Enqueue(Message &aMessage) { return Enqueue(aMessage, kQueuePositionTail); }
+    void Enqueue(Message &aMessage) { Enqueue(aMessage, kQueuePositionTail); }
 
     /**
      * This method adds a message at a given position (head/tail) of the queue.
@@ -690,14 +687,8 @@ public:
      * @param[in]  aMessage  The message to add.
      * @param[in]  aPosition The position (head or tail) where to add the message.
      *
-     * @retval OT_ERROR_NONE     Successfully added the message to the queue.
-     * @retval OT_ERROR_ALREADY  The message is already enqueued in a queue.
-     *
      */
-    otError Enqueue(Message &aMessage, QueuePosition aPosition)
-    {
-        return ot::MessageQueue::Enqueue(aMessage, aPosition);
-    }
+    void Enqueue(Message &aMessage, QueuePosition aPosition) { ot::MessageQueue::Enqueue(aMessage, aPosition); }
 
     /**
      * This method removes a message from the queue.
diff --git a/src/core/coap/coap_secure.cpp b/src/core/coap/coap_secure.cpp
index e06f8274e..fc3b91f42 100644
--- a/src/core/coap/coap_secure.cpp
+++ b/src/core/coap/coap_secure.cpp
@@ -176,13 +176,10 @@ otError CoapSecure::Send(ot::Message &aMessage, const Ip6::MessageInfo &aMessage
 {
     OT_UNUSED_VARIABLE(aMessageInfo);
 
-    otError error;
-
-    SuccessOrExit(error = mTransmitQueue.Enqueue(aMessage));
+    mTransmitQueue.Enqueue(aMessage);
     mTransmitTask.Post();
 
-exit:
-    return error;
+    return OT_ERROR_NONE;
 }
 
 void CoapSecure::HandleDtlsConnected(void *aContext, bool aConnected)
diff --git a/src/core/common/message.cpp b/src/core/common/message.cpp
index 07b4eeb1f..6302d4808 100644
--- a/src/core/common/message.cpp
+++ b/src/core/common/message.cpp
@@ -379,7 +379,7 @@ otError Message::SetPriority(uint8_t aPriority)
 
     if (priorityQueue != NULL)
     {
-        IgnoreError(priorityQueue->Enqueue(*this));
+        priorityQueue->Enqueue(*this);
     }
 
 exit:
@@ -801,16 +801,13 @@ Message *MessageQueue::GetHead(void) const
     return (GetTail() == NULL) ? NULL : GetTail()->Next();
 }
 
-otError MessageQueue::Enqueue(Message &aMessage, QueuePosition aPosition)
+void MessageQueue::Enqueue(Message &aMessage, QueuePosition aPosition)
 {
-    otError error = OT_ERROR_NONE;
-
-    VerifyOrExit(!aMessage.IsInAQueue(), error = OT_ERROR_ALREADY);
+    OT_ASSERT(!aMessage.IsInAQueue());
+    OT_ASSERT((aMessage.Next() == NULL) && (aMessage.Prev() == NULL));
 
     aMessage.SetMessageQueue(this);
 
-    OT_ASSERT((aMessage.Next() == NULL) && (aMessage.Prev() == NULL));
-
     if (GetTail() == NULL)
     {
         aMessage.Next() = &aMessage;
@@ -833,9 +830,6 @@ otError MessageQueue::Enqueue(Message &aMessage, QueuePosition aPosition)
             SetTail(&aMessage);
         }
     }
-
-exit:
-    return error;
 }
 
 otError MessageQueue::Dequeue(Message &aMessage)
@@ -944,14 +938,13 @@ Message *PriorityQueue::GetTail(void) const
     return FindFirstNonNullTail(0);
 }
 
-otError PriorityQueue::Enqueue(Message &aMessage)
+void PriorityQueue::Enqueue(Message &aMessage)
 {
-    otError  error = OT_ERROR_NONE;
     uint8_t  priority;
     Message *tail;
     Message *next;
 
-    VerifyOrExit(!aMessage.IsInAQueue(), error = OT_ERROR_ALREADY);
+    OT_ASSERT(!aMessage.IsInAQueue());
 
     aMessage.SetPriorityQueue(this);
 
@@ -975,9 +968,6 @@ otError PriorityQueue::Enqueue(Message &aMessage)
     }
 
     mTails[priority] = &aMessage;
-
-exit:
-    return error;
 }
 
 otError PriorityQueue::Dequeue(Message &aMessage)
diff --git a/src/core/common/message.hpp b/src/core/common/message.hpp
index 3e7521a23..54fcad3c3 100644
--- a/src/core/common/message.hpp
+++ b/src/core/common/message.hpp
@@ -943,11 +943,8 @@ public:
      *
      * @param[in]  aMessage  The message to add.
      *
-     * @retval OT_ERROR_NONE     Successfully added the message to the list.
-     * @retval OT_ERROR_ALREADY  The message is already enqueued in a list.
-     *
      */
-    otError Enqueue(Message &aMessage) { return Enqueue(aMessage, kQueuePositionTail); }
+    void Enqueue(Message &aMessage) { Enqueue(aMessage, kQueuePositionTail); }
 
     /**
      * This method adds a message at a given position (head/tail) of the list.
@@ -955,11 +952,8 @@ public:
      * @param[in]  aMessage  The message to add.
      * @param[in]  aPosition The position (head or tail) where to add the message.
      *
-     * @retval OT_ERROR_NONE     Successfully added the message to the list.
-     * @retval OT_ERROR_ALREADY  The message is already enqueued in a list.
-     *
      */
-    otError Enqueue(Message &aMessage, QueuePosition aPosition);
+    void Enqueue(Message &aMessage, QueuePosition aPosition);
 
     /**
      * This method removes a message from the list.
@@ -1040,11 +1034,8 @@ public:
      *
      * @param[in]  aMessage  The message to add.
      *
-     * @retval OT_ERROR_NONE     Successfully added the message to the list.
-     * @retval OT_ERROR_ALREADY  The message is already enqueued in a list.
-     *
      */
-    otError Enqueue(Message &aMessage);
+    void Enqueue(Message &aMessage);
 
     /**
      * This method removes a message from the list.
diff --git a/src/core/meshcop/joiner_router.cpp b/src/core/meshcop/joiner_router.cpp
index 227415457..ad9cd5a31 100644
--- a/src/core/meshcop/joiner_router.cpp
+++ b/src/core/meshcop/joiner_router.cpp
@@ -247,7 +247,7 @@ void JoinerRouter::DelaySendingJoinerEntrust(const Ip6::MessageInfo &aMessageInf
 
     SuccessOrExit(error = metadata.AppendTo(*message));
 
-    IgnoreError(mDelayedJoinEnts.Enqueue(*message));
+    mDelayedJoinEnts.Enqueue(*message);
 
     if (!mTimer.IsRunning())
     {
diff --git a/src/core/net/dns_client.cpp b/src/core/net/dns_client.cpp
index d087ef046..f5cded422 100644
--- a/src/core/net/dns_client.cpp
+++ b/src/core/net/dns_client.cpp
@@ -190,7 +190,7 @@ Message *Client::CopyAndEnqueueMessage(const Message &aMessage, const QueryMetad
 
     // Append the copy with retransmission data and add it to the queue.
     SuccessOrExit(error = aQueryMetadata.AppendTo(*messageCopy));
-    IgnoreError(mPendingQueries.Enqueue(*messageCopy));
+    mPendingQueries.Enqueue(*messageCopy);
 
     mRetransmissionTimer.FireAtIfEarlier(aQueryMetadata.mTransmissionTime);
 
diff --git a/src/core/net/ip6.cpp b/src/core/net/ip6.cpp
index b8fd12ab6..ecdda4341 100644
--- a/src/core/net/ip6.cpp
+++ b/src/core/net/ip6.cpp
@@ -448,7 +448,7 @@ exit:
 
 void Ip6::EnqueueDatagram(Message &aMessage)
 {
-    IgnoreError(mSendQueue.Enqueue(aMessage));
+    mSendQueue.Enqueue(aMessage);
     mSendQueueTask.Post();
 }
 
@@ -775,7 +775,7 @@ otError Ip6::HandleFragment(Message &aMessage, Netif *aNetif, MessageInfo &aMess
             mTimer.Start(kStateUpdatePeriod);
         }
 
-        IgnoreError(mReassemblyList.Enqueue(*message));
+        mReassemblyList.Enqueue(*message);
 
         otLogDebgIp6("start reassembly.");
     }
diff --git a/src/core/net/ip6_mpl.cpp b/src/core/net/ip6_mpl.cpp
index 0abb0df01..ee8f09ab1 100644
--- a/src/core/net/ip6_mpl.cpp
+++ b/src/core/net/ip6_mpl.cpp
@@ -326,7 +326,7 @@ void Mpl::AddBufferedMessage(Message &aMessage, uint16_t aSeedId, uint8_t aSeque
     metadata.GenerateNextTransmissionTime(TimerMilli::GetNow(), interval);
 
     SuccessOrExit(error = metadata.AppendTo(*messageCopy));
-    IgnoreError(mBufferedMessageSet.Enqueue(*messageCopy));
+    mBufferedMessageSet.Enqueue(*messageCopy);
 
     mRetransmissionTimer.FireAtIfEarlier(metadata.mTransmissionTime);
 
diff --git a/src/core/net/sntp_client.cpp b/src/core/net/sntp_client.cpp
index c6a4cda9b..a5d411123 100644
--- a/src/core/net/sntp_client.cpp
+++ b/src/core/net/sntp_client.cpp
@@ -195,7 +195,7 @@ Message *Client::CopyAndEnqueueMessage(const Message &aMessage, const QueryMetad
 
     // Append the copy with retransmission data and add it to the queue.
     SuccessOrExit(error = aQueryMetadata.AppendTo(*messageCopy));
-    IgnoreError(mPendingQueries.Enqueue(*messageCopy));
+    mPendingQueries.Enqueue(*messageCopy);
 
     mRetransmissionTimer.FireAtIfEarlier(aQueryMetadata.mTransmissionTime);
 
diff --git a/src/core/thread/mesh_forwarder.cpp b/src/core/thread/mesh_forwarder.cpp
index 6e24739c8..3560dd3df 100644
--- a/src/core/thread/mesh_forwarder.cpp
+++ b/src/core/thread/mesh_forwarder.cpp
@@ -261,7 +261,7 @@ Message *MeshForwarder::GetDirectTransmission(void)
 
         case OT_ERROR_ADDRESS_QUERY:
             IgnoreError(mSendQueue.Dequeue(*curMessage));
-            IgnoreError(mResolvingQueue.Enqueue(*curMessage));
+            mResolvingQueue.Enqueue(*curMessage);
             continue;
 
 #endif
@@ -1117,7 +1117,7 @@ void MeshForwarder::HandleFragment(const uint8_t *         aFrame,
             ClearReassemblyList();
         }
 
-        IgnoreError(mReassemblyList.Enqueue(*message));
+        mReassemblyList.Enqueue(*message);
 
         if (!mUpdateTimer.IsRunning())
         {
diff --git a/src/core/thread/mesh_forwarder_ftd.cpp b/src/core/thread/mesh_forwarder_ftd.cpp
index 61a15186d..bbe83ce34 100644
--- a/src/core/thread/mesh_forwarder_ftd.cpp
+++ b/src/core/thread/mesh_forwarder_ftd.cpp
@@ -52,7 +52,7 @@ otError MeshForwarder::SendMessage(Message &aMessage)
 
     IgnoreError(aMessage.SetOffset(0));
     aMessage.SetDatagramTag(0);
-    SuccessOrExit(error = mSendQueue.Enqueue(aMessage));
+    mSendQueue.Enqueue(aMessage);
 
     switch (aMessage.GetType())
     {
@@ -138,7 +138,6 @@ otError MeshForwarder::SendMessage(Message &aMessage)
 
     mScheduleTransmissionTask.Post();
 
-exit:
     return error;
 }
 
@@ -165,7 +164,7 @@ void MeshForwarder::HandleResolved(const Ip6::Address &aEid, otError aError)
 
             if (aError == OT_ERROR_NONE)
             {
-                IgnoreError(mSendQueue.Enqueue(*cur));
+                mSendQueue.Enqueue(*cur);
                 enqueuedMessage = true;
             }
             else
diff --git a/src/core/thread/mesh_forwarder_mtd.cpp b/src/core/thread/mesh_forwarder_mtd.cpp
index 5c072e0fe..dcd0c9ea8 100644
--- a/src/core/thread/mesh_forwarder_mtd.cpp
+++ b/src/core/thread/mesh_forwarder_mtd.cpp
@@ -39,17 +39,14 @@ namespace ot {
 
 otError MeshForwarder::SendMessage(Message &aMessage)
 {
-    otError error;
-
     aMessage.SetDirectTransmission();
     IgnoreError(aMessage.SetOffset(0));
     aMessage.SetDatagramTag(0);
 
-    SuccessOrExit(error = mSendQueue.Enqueue(aMessage));
+    mSendQueue.Enqueue(aMessage);
     mScheduleTransmissionTask.Post();
 
-exit:
-    return error;
+    return OT_ERROR_NONE;
 }
 
 otError MeshForwarder::EvictMessage(uint8_t aPriority)
diff --git a/src/core/thread/mle.cpp b/src/core/thread/mle.cpp
index 4e452a062..12a58dc58 100644
--- a/src/core/thread/mle.cpp
+++ b/src/core/thread/mle.cpp
@@ -2587,7 +2587,7 @@ otError Mle::AddDelayedResponse(Message &aMessage, const Ip6::Address &aDestinat
     metadata.mDestination = aDestination;
 
     SuccessOrExit(error = metadata.AppendTo(aMessage));
-    IgnoreError(mDelayedResponses.Enqueue(aMessage));
+    mDelayedResponses.Enqueue(aMessage);
 
     mDelayedResponseTimer.FireAtIfEarlier(metadata.mSendTime);
 
diff --git a/src/lib/spinel/spinel_buffer.cpp b/src/lib/spinel/spinel_buffer.cpp
index 2d9520fb5..f4895531c 100644
--- a/src/lib/spinel/spinel_buffer.cpp
+++ b/src/lib/spinel/spinel_buffer.cpp
@@ -403,7 +403,7 @@ otError Buffer::InFrameFeedMessage(otMessage *aMessage)
     SuccessOrExit(error = InFrameBeginSegment());
 
     // Enqueue the message in the current write frame queue.
-    SuccessOrExit(error = otMessageQueueEnqueue(&mWriteFrameMessageQueue, aMessage));
+    otMessageQueueEnqueue(&mWriteFrameMessageQueue, aMessage);
 
     // End/Close the current segment marking the flag that it contains an associated message.
     InFrameEndSegment(kSegmentHeaderMessageIndicatorFlag);
@@ -521,7 +521,7 @@ otError Buffer::InFrameEnd(void)
     while ((message = otMessageQueueGetHead(&mWriteFrameMessageQueue)) != NULL)
     {
         IgnoreError(otMessageQueueDequeue(&mWriteFrameMessageQueue, message));
-        IgnoreError(otMessageQueueEnqueue(&mMessageQueue[mWriteDirection], message));
+        otMessageQueueEnqueue(&mMessageQueue[mWriteDirection], message);
     }
 #endif
 
diff --git a/src/ncp/ncp_base_mtd.cpp b/src/ncp/ncp_base_mtd.cpp
index a8b06496d..b0854a9a7 100644
--- a/src/ncp/ncp_base_mtd.cpp
+++ b/src/ncp/ncp_base_mtd.cpp
@@ -3397,7 +3397,7 @@ void NcpBase::HandleDatagramFromStack(otMessage *aMessage)
 {
     VerifyOrExit(aMessage != NULL, OT_NOOP);
 
-    SuccessOrExit(otMessageQueueEnqueue(&mMessageQueue, aMessage));
+    otMessageQueueEnqueue(&mMessageQueue, aMessage);
 
     // If there is no queued spinel command response, try to write/send
     // the datagram message immediately. If there is a queued response
@@ -3462,7 +3462,7 @@ otError NcpBase::SendQueuedDatagramMessages(void)
 
         if (error != OT_ERROR_NONE)
         {
-            IgnoreError(otMessageQueueEnqueueAtHead(&mMessageQueue, message));
+            otMessageQueueEnqueueAtHead(&mMessageQueue, message);
         }
 
         SuccessOrExit(error);
diff --git a/tests/unit/test_message_queue.cpp b/tests/unit/test_message_queue.cpp
index 911b07c5a..890da4576 100644
--- a/tests/unit/test_message_queue.cpp
+++ b/tests/unit/test_message_queue.cpp
@@ -96,28 +96,27 @@ void TestMessageQueue(void)
     VerifyMessageQueueContent(messageQueue, 0);
 
     // Enqueue 1 message and remove it
-    SuccessOrQuit(messageQueue.Enqueue(*msg[0]), "MessageQueue::Enqueue() failed.");
+    messageQueue.Enqueue(*msg[0]);
     VerifyMessageQueueContent(messageQueue, 1, msg[0]);
     SuccessOrQuit(messageQueue.Dequeue(*msg[0]), "MessageQueue::Dequeue() failed.");
     VerifyMessageQueueContent(messageQueue, 0);
 
     // Enqueue 1 message at head and remove it
-    SuccessOrQuit(messageQueue.Enqueue(*msg[0], ot::MessageQueue::kQueuePositionHead),
-                  "MessageQueue::Enqueue() failed.");
+    messageQueue.Enqueue(*msg[0], ot::MessageQueue::kQueuePositionHead);
     VerifyMessageQueueContent(messageQueue, 1, msg[0]);
     SuccessOrQuit(messageQueue.Dequeue(*msg[0]), "MessageQueue::Dequeue() failed.");
     VerifyMessageQueueContent(messageQueue, 0);
 
     // Enqueue 5 messages
-    SuccessOrQuit(messageQueue.Enqueue(*msg[0]), "MessageQueue::Enqueue() failed.");
+    messageQueue.Enqueue(*msg[0]);
     VerifyMessageQueueContent(messageQueue, 1, msg[0]);
-    SuccessOrQuit(messageQueue.Enqueue(*msg[1]), "MessageQueue::Enqueue() failed.");
+    messageQueue.Enqueue(*msg[1]);
     VerifyMessageQueueContent(messageQueue, 2, msg[0], msg[1]);
-    SuccessOrQuit(messageQueue.Enqueue(*msg[2]), "MessageQueue::Enqueue() failed.");
+    messageQueue.Enqueue(*msg[2]);
     VerifyMessageQueueContent(messageQueue, 3, msg[0], msg[1], msg[2]);
-    SuccessOrQuit(messageQueue.Enqueue(*msg[3]), "MessageQueue::Enqueue() failed.");
+    messageQueue.Enqueue(*msg[3]);
     VerifyMessageQueueContent(messageQueue, 4, msg[0], msg[1], msg[2], msg[3]);
-    SuccessOrQuit(messageQueue.Enqueue(*msg[4]), "MessageQueue::Enqueue() failed.");
+    messageQueue.Enqueue(*msg[4]);
     VerifyMessageQueueContent(messageQueue, 5, msg[0], msg[1], msg[2], msg[3], msg[4]);
 
     // Check the GetInfo()
@@ -137,9 +136,9 @@ void TestMessageQueue(void)
     VerifyMessageQueueContent(messageQueue, 2, msg[1], msg[2]);
 
     // Add after remove
-    SuccessOrQuit(messageQueue.Enqueue(*msg[0]), "MessageQueue::Enqueue() failed.");
+    messageQueue.Enqueue(*msg[0]);
     VerifyMessageQueueContent(messageQueue, 3, msg[1], msg[2], msg[0]);
-    SuccessOrQuit(messageQueue.Enqueue(*msg[3]), "MessageQueue::Enqueue() failed.");
+    messageQueue.Enqueue(*msg[3]);
     VerifyMessageQueueContent(messageQueue, 4, msg[1], msg[2], msg[0], msg[3]);
 
     // Remove from middle
@@ -147,8 +146,7 @@ void TestMessageQueue(void)
     VerifyMessageQueueContent(messageQueue, 3, msg[1], msg[0], msg[3]);
 
     // Add to head
-    SuccessOrQuit(messageQueue.Enqueue(*msg[2], ot::MessageQueue::kQueuePositionHead),
-                  "MessageQueue::Enqueue() failed.");
+    messageQueue.Enqueue(*msg[2], ot::MessageQueue::kQueuePositionHead);
     VerifyMessageQueueContent(messageQueue, 4, msg[2], msg[1], msg[0], msg[3]);
 
     // Remove from head
@@ -160,13 +158,11 @@ void TestMessageQueue(void)
     VerifyMessageQueueContent(messageQueue, 2, msg[0], msg[3]);
 
     // Add to head
-    SuccessOrQuit(messageQueue.Enqueue(*msg[1], ot::MessageQueue::kQueuePositionHead),
-                  "MessageQueue::Enqueue() failed.");
+    messageQueue.Enqueue(*msg[1], ot::MessageQueue::kQueuePositionHead);
     VerifyMessageQueueContent(messageQueue, 3, msg[1], msg[0], msg[3]);
 
     // Add to tail
-    SuccessOrQuit(messageQueue.Enqueue(*msg[2], ot::MessageQueue::kQueuePositionTail),
-                  "MessageQueue::Enqueue() failed.");
+    messageQueue.Enqueue(*msg[2], ot::MessageQueue::kQueuePositionTail);
     VerifyMessageQueueContent(messageQueue, 4, msg[1], msg[0], msg[3], msg[2]);
 
     // Remove all messages.
@@ -179,11 +175,9 @@ void TestMessageQueue(void)
     SuccessOrQuit(messageQueue.Dequeue(*msg[0]), "MessageQueue::Dequeue() failed.");
     VerifyMessageQueueContent(messageQueue, 0);
 
-    // Check the failure cases: Enqueue an already queued message or dequeue a message not in the queue.
-    SuccessOrQuit(messageQueue.Enqueue(*msg[0]), "MessageQueue::Enqueue() failed.");
+    // Check the failure cases: Dequeue a message not in the queue.
+    messageQueue.Enqueue(*msg[0]);
     VerifyMessageQueueContent(messageQueue, 1, msg[0]);
-    error = messageQueue.Enqueue(*msg[0]);
-    VerifyOrQuit(error == OT_ERROR_ALREADY, "Enqueuing an already queued message did not fail as expected.");
     error = messageQueue.Dequeue(*msg[1]);
     VerifyOrQuit(error == OT_ERROR_NOT_FOUND, "Dequeuing a message not in the queue did not fail as expected.");
 
@@ -246,13 +240,13 @@ void TestMessageQueueOtApis(void)
     VerifyMessageQueueContentUsingOtApi(&queue, 0);
 
     // Add message to the queue and check the content
-    SuccessOrQuit(otMessageQueueEnqueue(&queue, msg[0]), "Failed to enqueue a message to otMessageQueue.");
+    otMessageQueueEnqueue(&queue, msg[0]);
     VerifyMessageQueueContentUsingOtApi(&queue, 1, msg[0]);
-    SuccessOrQuit(otMessageQueueEnqueue(&queue, msg[1]), "Failed to enqueue a message to otMessageQueue.");
+    otMessageQueueEnqueue(&queue, msg[1]);
     VerifyMessageQueueContentUsingOtApi(&queue, 2, msg[0], msg[1]);
-    SuccessOrQuit(otMessageQueueEnqueueAtHead(&queue, msg[2]), "Failed to enqueue a message to otMessageQueue.");
+    otMessageQueueEnqueueAtHead(&queue, msg[2]);
     VerifyMessageQueueContentUsingOtApi(&queue, 3, msg[2], msg[0], msg[1]);
-    SuccessOrQuit(otMessageQueueEnqueue(&queue, msg[3]), "Failed to enqueue a message to otMessageQueue.");
+    otMessageQueueEnqueue(&queue, msg[3]);
     VerifyMessageQueueContentUsingOtApi(&queue, 4, msg[2], msg[0], msg[1], msg[3]);
 
     // Remove elements and check the content
@@ -264,8 +258,6 @@ void TestMessageQueueOtApis(void)
     VerifyMessageQueueContentUsingOtApi(&queue, 1, msg[2]);
 
     // Check the expected failure cases for the enqueue and dequeue:
-    error = otMessageQueueEnqueue(&queue, msg[2]);
-    VerifyOrQuit(error == OT_ERROR_ALREADY, "Enqueuing an already queued message did not fail as expected.");
     error = otMessageQueueDequeue(&queue, msg[0]);
     VerifyOrQuit(error == OT_ERROR_NOT_FOUND, "Dequeuing a message not in the queue did not fail as expected.");
 
@@ -276,9 +268,9 @@ void TestMessageQueueOtApis(void)
     VerifyOrQuit(message == NULL, "otMessageQueueGetNext() did not return NULL for a message not in the queue.");
 
     // Check the failure case when attempting to do otMessageQueueGetNext() but passing in a wrong queue pointer.
-    SuccessOrQuit(otMessageQueueEnqueue(&queue2, msg[0]), "Failed to enqueue a message to otMessageQueue.");
+    otMessageQueueEnqueue(&queue2, msg[0]);
     VerifyMessageQueueContentUsingOtApi(&queue2, 1, msg[0]);
-    SuccessOrQuit(otMessageQueueEnqueue(&queue2, msg[1]), "Failed to enqueue a message to otMessageQueue.");
+    otMessageQueueEnqueue(&queue2, msg[1]);
     VerifyMessageQueueContentUsingOtApi(&queue2, 2, msg[0], msg[1]);
 
     message = otMessageQueueGetNext(&queue2, msg[0]);
diff --git a/tests/unit/test_priority_queue.cpp b/tests/unit/test_priority_queue.cpp
index 914db28e4..fb21ce9c2 100644
--- a/tests/unit/test_priority_queue.cpp
+++ b/tests/unit/test_priority_queue.cpp
@@ -210,22 +210,22 @@ void TestPriorityQueue(void)
     VerifyPriorityQueueContent(queue, 0);
 
     // Add msgs in different orders and check the content of queue.
-    SuccessOrQuit(queue.Enqueue(*msgHigh[0]), "PriorityQueue::Enqueue() failed.");
+    queue.Enqueue(*msgHigh[0]);
     VerifyPriorityQueueContent(queue, 1, msgHigh[0]);
-    SuccessOrQuit(queue.Enqueue(*msgHigh[1]), "PriorityQueue::Enqueue() failed.");
+    queue.Enqueue(*msgHigh[1]);
     VerifyPriorityQueueContent(queue, 2, msgHigh[0], msgHigh[1]);
-    SuccessOrQuit(queue.Enqueue(*msgNet[0]), "PriorityQueue::Enqueue() failed.");
+    queue.Enqueue(*msgNet[0]);
     VerifyPriorityQueueContent(queue, 3, msgNet[0], msgHigh[0], msgHigh[1]);
-    SuccessOrQuit(queue.Enqueue(*msgNet[1]), "PriorityQueue::Enqueue() failed.");
+    queue.Enqueue(*msgNet[1]);
     VerifyPriorityQueueContent(queue, 4, msgNet[0], msgNet[1], msgHigh[0], msgHigh[1]);
-    SuccessOrQuit(queue.Enqueue(*msgHigh[2]), "PriorityQueue::Enqueue() failed.");
+    queue.Enqueue(*msgHigh[2]);
     VerifyPriorityQueueContent(queue, 5, msgNet[0], msgNet[1], msgHigh[0], msgHigh[1], msgHigh[2]);
-    SuccessOrQuit(queue.Enqueue(*msgLow[0]), "PriorityQueue::Enqueue() failed.");
+    queue.Enqueue(*msgLow[0]);
     VerifyPriorityQueueContent(queue, 6, msgNet[0], msgNet[1], msgHigh[0], msgHigh[1], msgHigh[2], msgLow[0]);
-    SuccessOrQuit(queue.Enqueue(*msgNor[0]), "PriorityQueue::Enqueue() failed.");
+    queue.Enqueue(*msgNor[0]);
     VerifyPriorityQueueContent(queue, 7, msgNet[0], msgNet[1], msgHigh[0], msgHigh[1], msgHigh[2], msgNor[0],
                                msgLow[0]);
-    SuccessOrQuit(queue.Enqueue(*msgHigh[3]), "PriorityQueue::Enqueue() failed.");
+    queue.Enqueue(*msgHigh[3]);
     VerifyPriorityQueueContent(queue, 8, msgNet[0], msgNet[1], msgHigh[0], msgHigh[1], msgHigh[2], msgHigh[3],
                                msgNor[0], msgLow[0]);
 
@@ -248,22 +248,20 @@ void TestPriorityQueue(void)
     SuccessOrQuit(queue.Dequeue(*msgHigh[3]), "PriorityQueue::Dequeue() failed.");
     VerifyPriorityQueueContent(queue, 0);
 
-    // Check the failure cases: Enqueuing an already queued message, or dequeuing a message not queued.
-    SuccessOrQuit(queue.Enqueue(*msgNet[0]), "PriorityQueue::Enqueue() failed.");
+    // Check the failure cases: Dequeuing a message not queued.
+    queue.Enqueue(*msgNet[0]);
     VerifyPriorityQueueContent(queue, 1, msgNet[0]);
-    VerifyOrQuit(queue.Enqueue(*msgNet[0]) == OT_ERROR_ALREADY,
-                 "Enqueuing an already queued message did not fail as expected.");
     VerifyOrQuit(queue.Dequeue(*msgHigh[0]) == OT_ERROR_NOT_FOUND,
                  "Dequeuing a message not queued, did not fail as expected.");
     SuccessOrQuit(queue.Dequeue(*msgNet[0]), "PriorityQueue::Dequeue() failed.");
     VerifyPriorityQueueContent(queue, 0);
 
     // Change the priority of an already queued message and check the order change in the queue.
-    SuccessOrQuit(queue.Enqueue(*msgNor[0]), "PriorityQueue::Enqueue() failed.");
+    queue.Enqueue(*msgNor[0]);
     VerifyPriorityQueueContent(queue, 1, msgNor[0]);
-    SuccessOrQuit(queue.Enqueue(*msgHigh[0]), "PriorityQueue::Enqueue() failed.");
+    queue.Enqueue(*msgHigh[0]);
     VerifyPriorityQueueContent(queue, 2, msgHigh[0], msgNor[0]);
-    SuccessOrQuit(queue.Enqueue(*msgLow[0]), "PriorityQueue::Enqueue() failed.");
+    queue.Enqueue(*msgLow[0]);
     VerifyPriorityQueueContent(queue, 3, msgHigh[0], msgNor[0], msgLow[0]);
 
     SuccessOrQuit(msgNor[0]->SetPriority(ot::Message::kPriorityNet),
@@ -287,9 +285,9 @@ void TestPriorityQueue(void)
                   "SetPriority failed for an already queued message.");
     VerifyPriorityQueueContent(queue, 3, msgHigh[0], msgNor[0], msgLow[0]);
 
-    SuccessOrQuit(messageQueue.Enqueue(*msgNor[1]), "MessageQueue::Enqueue() failed.");
-    SuccessOrQuit(messageQueue.Enqueue(*msgHigh[1]), "MessageQueue::Enqueue() failed.");
-    SuccessOrQuit(messageQueue.Enqueue(*msgNet[1]), "MessageQueue::Enqueue() failed.");
+    messageQueue.Enqueue(*msgNor[1]);
+    messageQueue.Enqueue(*msgHigh[1]);
+    messageQueue.Enqueue(*msgNet[1]);
     VerifyMsgQueueContent(messageQueue, 3, msgNor[1], msgHigh[1], msgNet[1]);
 
     // Change priority of message and check for not in messageQueue.