diff --git a/include/mbedtls/ctr_drbg.h b/include/mbedtls/ctr_drbg.h
index 9a3aba0..10f9389 100644
--- a/include/mbedtls/ctr_drbg.h
+++ b/include/mbedtls/ctr_drbg.h
@@ -239,16 +239,15 @@
 /**
  * \brief              This function updates the state of the CTR_DRBG context.
  *
- * \note               If \p add_len is greater than
- *                     #MBEDTLS_CTR_DRBG_MAX_SEED_INPUT, only the first
- *                     #MBEDTLS_CTR_DRBG_MAX_SEED_INPUT Bytes are used.
- *                     The remaining Bytes are silently discarded.
- *
  * \param ctx          The CTR_DRBG context.
  * \param additional   The data to update the state with.
- * \param add_len      Length of \p additional data.
+ * \param add_len      Length of \p additional in bytes. This must be at
+ *                     most #MBEDTLS_CTR_DRBG_MAX_SEED_INPUT.
  *
  * \return             \c 0 on success.
+ * \return             #MBEDTLS_ERR_CTR_DRBG_INPUT_TOO_BIG if
+ *                     \p add_len is more than
+ *                     #MBEDTLS_CTR_DRBG_MAX_SEED_INPUT.
  * \return             An error from the underlying AES cipher on failure.
  */
 int mbedtls_ctr_drbg_update_ret( mbedtls_ctr_drbg_context *ctx,