diff --git a/include/mbedtls/ctr_drbg.h b/include/mbedtls/ctr_drbg.h
index d8ef5d095..6c099adf4 100644
--- a/include/mbedtls/ctr_drbg.h
+++ b/include/mbedtls/ctr_drbg.h
@@ -284,6 +284,15 @@ void mbedtls_ctr_drbg_init( mbedtls_ctr_drbg_context *ctx );
  *                      device.
  */
 #endif
+#if defined(MBEDTLS_THREADING_C)
+/**
+ * \note                When Mbed TLS is built with threading support,
+ *                      after this function returns successfully,
+ *                      it is safe to call mbedtls_ctr_drbg_random()
+ *                      from multiple threads. Other operations, including
+ *                      reseeding, are not thread-safe.
+ */
+#endif /* MBEDTLS_THREADING_C */
 /**
  * \param ctx           The CTR_DRBG context to seed.
  *                      It must have been initialized with
@@ -293,6 +302,8 @@ void mbedtls_ctr_drbg_init( mbedtls_ctr_drbg_context *ctx );
  *                      the same context unless you call
  *                      mbedtls_ctr_drbg_free() and mbedtls_ctr_drbg_init()
  *                      again first.
+ *                      After a failed call to mbedtls_ctr_drbg_seed(),
+ *                      you must call mbedtls_ctr_drbg_free().
  * \param f_entropy     The entropy callback, taking as arguments the
  *                      \p p_entropy context, the buffer to fill, and the
  *                      length of the buffer.
@@ -384,6 +395,11 @@ void mbedtls_ctr_drbg_set_reseed_interval( mbedtls_ctr_drbg_context *ctx,
  * \brief               This function reseeds the CTR_DRBG context, that is
  *                      extracts data from the entropy source.
  *
+ * \note                This function is not thread-safe. It is not safe
+ *                      to call this function if another thread might be
+ *                      concurrently obtaining random numbers from the same
+ *                      context or updating or reseeding the same context.
+ *
  * \param ctx           The CTR_DRBG context.
  * \param additional    Additional data to add to the state. Can be \c NULL.
  * \param len           The length of the additional data.
@@ -401,6 +417,11 @@ int mbedtls_ctr_drbg_reseed( mbedtls_ctr_drbg_context *ctx,
 /**
  * \brief              This function updates the state of the CTR_DRBG context.
  *
+ * \note               This function is not thread-safe. It is not safe
+ *                     to call this function if another thread might be
+ *                     concurrently obtaining random numbers from the same
+ *                     context or updating or reseeding the same context.
+ *
  * \param ctx          The CTR_DRBG context.
  * \param additional   The data to update the state with. This must not be
  *                     \c NULL unless \p add_len is \c 0.
@@ -424,6 +445,11 @@ int mbedtls_ctr_drbg_update_ret( mbedtls_ctr_drbg_context *ctx,
  * This function automatically reseeds if the reseed counter is exceeded
  * or prediction resistance is enabled.
  *
+ * \note                This function is not thread-safe. It is not safe
+ *                      to call this function if another thread might be
+ *                      concurrently obtaining random numbers from the same
+ *                      context or updating or reseeding the same context.
+ *
  * \param p_rng         The CTR_DRBG context. This must be a pointer to a
  *                      #mbedtls_ctr_drbg_context structure.
  * \param output        The buffer to fill.
@@ -452,8 +478,16 @@ int mbedtls_ctr_drbg_random_with_add( void *p_rng,
  *
  * This function automatically reseeds if the reseed counter is exceeded
  * or prediction resistance is enabled.
- *
- *
+ */
+#if defined(MBEDTLS_THREADING_C)
+/**
+ * \note                When Mbed TLS is built with threading support,
+ *                      it is safe to call mbedtls_ctr_drbg_random()
+ *                      from multiple threads. Other operations, including
+ *                      reseeding, are not thread-safe.
+ */
+#endif /* MBEDTLS_THREADING_C */
+/**
  * \param p_rng         The CTR_DRBG context. This must be a pointer to a
  *                      #mbedtls_ctr_drbg_context structure.
  * \param output        The buffer to fill.