Comment by Broken_Hippo

OTC just means "Over the counter" or "without a prescription". A good way to test this is to see if your health insurance in the US will pay for it - most won't pay for OTC drugs.

Behind the counter just means there is less chance of folks stealing it and more control over who buys it and the amounts they buy.

There is generally a good amount of things you can get at the pharmacy that are like this: Most of the time, they are simply ordered if someone wants them because there isn't enough demand to keep it on the shelf. Most require no ID either: Sweet almond oil (for ears) is the example I can think of.

Related: In some states, they require a prescription for it because their laws are stricter than the federal guidelines.