Sleep syrup is a form of medicine used in Panem. As its name suggests, the syrup works as an anesthetic, sedating patients so that their injuries can be more easily attended to. Sleep syrup has a distinctively sweet taste and is relatively inexpensive compared to other medicines, but is also highly addictive (somewhat similar to many real-life pain medications).
During the 74th Hunger Games, Katniss received a small vial of sleep syrup as a gift from a sponsor. At the time, Peeta was suffering from a severe leg injury inflicted by Cato. A feast had been announced for the remaining tributes, since they all needed a particular item at that moment. While there was a chance of Katniss getting medicine to help Peeta at this feast, he didn’t want her risking her life against the other tributes just to save him. By tricking Peeta into consuming the sleep syrup, Katniss was able to retrieve the medicine and save his life without interference.