SSRI antidepressants (Selective Serotonin Reuptake Inhibitors, the best known being Prozactm) are amongst the most commonly taken medicines. However, there seems to be no way of knowing in advance whether or not SSRIs will work effectively. Now a group of researchers has developed a new theory of SSRI action, and tested it in stressed mice. The results show why the circumstances we find ourselves in may influence whether an antidepressant works or not.