The Regimen is a series of drugs developed for Division by Joseph Mars. The Guardians use the drugs to enhance their physical abilities. However, when the the regimen was discontinued, it was discovered that Guardians suffering from Regimen withdraw began showing erratic behavior, some were psychotically obsessive, depending on their personal drive. This causes the person going through the withdrawals to want to pursuit their top goal without regard for the consequences. Owen for example, wanted to take Division down and the withdrawal made him want to do it by uploading the black boxes, which would have caused more damage than fix. Percy kept a stash for the Guardians.
The Regime enables the user to sleep less, fight longer, faster, and harder. As a testament of their effectiveness, Owen (after he got clean) and Michael fought Jeff Shore, a Guardian, and despite being 2 on 1 and their Division training, they barely managed to kill him.
Owen Elliot and Dana Winters are the only people who has successfully weaned themselves off the Regimen.