diff --git a/news/6794.doc b/news/6794.doc
new file mode 100644
index 00000000000..55bc01404dd
--- /dev/null
+++ b/news/6794.doc
@@ -0,0 +1 @@
+Document that ``--ignore-installed`` is dangerous.
diff --git a/src/pip/_internal/commands/install.py b/src/pip/_internal/commands/install.py
index 927e7817be2..e7e16809646 100644
--- a/src/pip/_internal/commands/install.py
+++ b/src/pip/_internal/commands/install.py
@@ -187,7 +187,11 @@ def __init__(self, *args, **kw):
             '-I', '--ignore-installed',
             dest='ignore_installed',
             action='store_true',
-            help='Ignore the installed packages (reinstalling instead).')
+            help='Ignore the installed packages, overwriting them. '
+                 'This can break your system if the existing package '
+                 'is of a different version or was installed '
+                 'with a different package manager!'
+        )
 
         cmd_opts.add_option(cmdoptions.ignore_requires_python())
         cmd_opts.add_option(cmdoptions.no_build_isolation())