The syringes of illegal drugs, displayed in the top right corner of the view of the passenger seat of the car, can be activated by clicking on them. They will temporalily restore sanity, colouring the sanity bar yellow. This temporary sanity boost will dissipate over several in game hours, although too slowly to often be noticeable while exploring a dungeon. Frequent use of the drug will reduce its effectiveness.
Additional doses of illegal drugs can be bought for 15 pounds at hospitals in un-corrupted towns, or for 5 pounds through the Old Drug Dealer random encounter.