diff --git a/tox.ini b/tox.ini
index d1bb45a..653a310 100644
--- a/tox.ini
+++ b/tox.ini
@@ -14,7 +14,7 @@ setenv = VIRTUAL_ENV={envdir}
          JUJU_REPOSITORY={envdir}/tmp/build
 passenv = http_proxy https_proxy
 install_command =
-  pip install {opts} {packages}
+  pip install --no-cache-dir {opts} {packages}
 deps =
   -r{toxinidir}/requirements.txt
 whitelist_externals = /bin/true /bin/echo /bin/mkdir /bin/ln