OTC in health care stands for over the counter. Certain drugs can be bought over the counter. This means that you do not need a prescription to get the drugs. You walk into a pharmacy and take the drugs off of the shelf and purchase them at the cash register. Prescription drugs, on the other hand, require a prescription from a doctor.
Most of the time, if not all of the time, you would need to go to the doctor and have him, or she looks you over before writing you a prescription for a medicine that you have not taken before. Then the doctor will give you a piece of paper to take to the pharmacy. If you do not have one for that prescription drug, the pharmacist will not fill the medicine for you.