An over-the-counter drug is one you can buy without a prescription. Aspirin is an over-the-counter drug, but for antibiotics or other prescription drugs, you'll have to see the pharmacist.
Over-the-counter drugs can be bought without a doctor's permission. You don't even have to go to the pharmacy to get over-the-counter drugs, just march up to the counter and buy it. There are all kinds of over-the-counter drugs, like pain relievers, allergy medicine, or various types of heartburn relief. Generally, over-the-counter drugs are less potentially dangerous than prescription drugs, so they’re easier to buy.