diff --git a/libavcodec/avcodec.h b/libavcodec/avcodec.h
index 982b5a8373..5e064327b4 100644
--- a/libavcodec/avcodec.h
+++ b/libavcodec/avcodec.h
@@ -3655,6 +3655,9 @@ void avcodec_free_frame(AVFrame **frame);
  *
  * @warning This function is not thread safe!
  *
+ * @note Always call this function before using decoding routines (such as
+ * @ref avcodec_decode_video2()).
+ *
  * @code
  * avcodec_register_all();
  * av_dict_set(&opts, "b", "2.5M", 0);
@@ -4103,6 +4106,9 @@ attribute_deprecated int avcodec_decode_audio3(AVCodecContext *avctx, int16_t *s
  *          larger than the actual read bytes because some optimized bitstream
  *          readers read 32 or 64 bits at once and could read over the end.
  *
+ * @note The AVCodecContext MUST have been opened with @ref avcodec_open2()
+ * before packets may be fed to the decoder.
+ *
  * @param      avctx the codec context
  * @param[out] frame The AVFrame in which to store decoded audio samples.
  *                   The decoder will allocate a buffer for the decoded frame by
@@ -4147,6 +4153,9 @@ int avcodec_decode_audio4(AVCodecContext *avctx, AVFrame *frame,
  * between input and output, these need to be fed with avpkt->data=NULL,
  * avpkt->size=0 at the end to return the remaining frames.
  *
+ * @note The AVCodecContext MUST have been opened with @ref avcodec_open2()
+ * before packets may be fed to the decoder.
+ *
  * @param avctx the codec context
  * @param[out] picture The AVFrame in which the decoded video frame will be stored.
  *             Use av_frame_alloc() to get an AVFrame. The codec will
@@ -4193,6 +4202,9 @@ int avcodec_decode_video2(AVCodecContext *avctx, AVFrame *picture,
  * returning subtitles. It is safe to flush even those decoders that are not
  * marked with CODEC_CAP_DELAY, then no subtitles will be returned.
  *
+ * @note The AVCodecContext MUST have been opened with @ref avcodec_open2()
+ * before packets may be fed to the decoder.
+ *
  * @param avctx the codec context
  * @param[out] sub The Preallocated AVSubtitle in which the decoded subtitle will be stored,
  *                 must be freed with avsubtitle_free if *got_sub_ptr is set.