Medicine is sold in two ways:

• "Prescription drugs" require a prescription from a doctor. You can't buy them unless you have this. They're only sold at drug stores.
• "Over-the-counter drugs" don't require a prescription. You can buy them at a drug store or sometimes even at grocery stores, convenience stores, etc.

For over-the-counter drugs, you can say that they are "sold over the counter":

A: Is it sold over the counter?

B: No, it's a prescription drug.