Idioms of counter in English

over the counter

goods, especially medicines, for sale over the counter can be bought without a prescription (= written permission from a doctor to buy a medicine) or special licence

These tablets are available over the counter.

This kind of medication cannot be bought over the counter.

under the counter

goods that are bought or sold under the counter are sold secretly and sometimes illegally

Pornography may be legally banned but it is still available under the counter.