diff --git a/reference/misc/functions/uniqid.xml b/reference/misc/functions/uniqid.xml
index 57f6c34705..b7a7b3a4aa 100644
--- a/reference/misc/functions/uniqid.xml
+++ b/reference/misc/functions/uniqid.xml
@@ -18,6 +18,16 @@
    microseconds.
   </para>
   &caution.cryptographically-insecure;
+  <warning>
+    <para>
+     This function does not guarantee uniqueness of return
+     value. Since most systems adjust system clock by NTP or like,
+     system time is changed constantly. Therefore, it is possible that
+     this function does not return unique ID for the
+     process/thread. Use <parameter>more_entropy</parameter> to
+     increase likelihood of uniqueness.
+    </para>
+  </warning>
  </refsect1>
 
  <refsect1 role="parameters">
@@ -57,8 +67,14 @@
  <refsect1 role="returnvalues">
   &reftitle.returnvalues;
   <para>
-   Returns the unique identifier, as a string.
+   Returns timestamp based unique identifier as a string.
   </para>
+  <warning>
+   <para>
+     This function tries to create unique identifier, but it does not
+     guarantee 100% uniqueness of return value.
+   </para>
+  </warning>
  </refsect1>
 
  <refsect1 role="examples">