This season takes place about three years after the initial outbreak, at this point most medications, especially antibiotics and opioid painkillers like morphine, that are not stored properly (in a cool, dark, dry room) would have lost most of their potency and would be useless. The pills on the floor of that room Gabriel finds were exposed to sunlight and fluctuating temperatures and humidity over a period of years, which will cause most medications to decay rapidly. If properly stored most medications can actually last for years beyond the expiration date, FDA guidelines for stockpiled medications stored properly says most medications are safe to use 6 years past their expiration date. So at this point in time the only medications that would still have any clinical effect would be ones that were stored in an air tight, pitch black, temperature controlled room.