diff --git a/man3/memcmp.3 b/man3/memcmp.3
index de93d22dc..0d053e6d7 100644
--- a/man3/memcmp.3
+++ b/man3/memcmp.3
@@ -68,6 +68,12 @@ The
 function is thread-safe.
 .SH CONFORMING TO
 SVr4, 4.3BSD, C89, C99, POSIX.1-2001.
+.SH NOTES
+.PP
+Do not use
+.BR memcmp ()
+to compare security critical data, such as cryptographic secrets,
+because the required CPU time depends on the amount of equal bytes.
 .SH SEE ALSO
 .BR bcmp (3),
 .BR strcasecmp (3),