Over-the-counter drug
Over-the-counter (OTC) drugs are medicines that may be sold without a prescription, in contrast to prescription drugs. 
The name "over-the-counter" is somewhat confusing to some, since these items can be found on the shelves of stores and bought like any other packaged product in some countries or in others may be bought "over the counter" from the pharmacy, while prescription drugs are sold at a pharmacy counter.  The term likely dates back to before self service shopping became common, when most goods were obtained by requesting them from a clerk at a sales counter; while prescription drugs required a visit to the doctor first, these drugs could be purchased "over the (sales) counter" just like other goods. Some medicines considered safe in general terms may be available in general stores, supermarkets, gas stations etc. The rules vary considerably from country to country.

# Regulation by country

## United States
In the United States, the manufacture and sale of OTC substances is regulated by the  FDA.  The Federal Food, Drug, and Cosmetic Act requires that all "new drugs" are required to obtain a New Drug Application ("NDA") prior to entering interstate commerce, but the act exempts any drugs generally recognized as safe and effective ("GRAS/E") from this requirement.  In order to fit the vast amount of OTC drugs that were already on the market prior to the requirement that all drugs obtain an NDA, the FDA created the OTC monograph system to review classes of drugs and categorize them as GRAS/E by regulation.  This meant that certain classes of OTC drugs were not required to obtain an NDA and could remain on the market.
Thus, manufacture must be done either pursuant to an FDA monograph, which specifies types of OTC drugs, active ingredients and labeling requirements, or pursuant to a New Drug Application (NDA), for products which do not fit within a specific monograph.  Because an NDA is extremely expensive to obtain, due primarily to testing requirements, most OTC substances produced in the USA are sunscreens, anti-microbial and anti-fungal products, external and internal analgesics such as lidocaine and aspirin, psoriasis and eczema topical treatments, anti-dandruff shampoos containing coal tar, and other topical products with a therapeutic effect.
The Federal Trade Commission regulates advertising of OTC products.  This is in contrast to prescription drug advertising, which is regulated by the FDA.

## United Kingdom
In the United Kingdom, items may be restricted to sale in three ways:
- Pharmacy dispensing only, with a prescription.
- Dispensing only with supervision of a pharmacist.
- No restrictions, available off the shelf. (Quantity restricitons apply)
In the United Kingdom, over-the-counter medicines are marked with (P). They appear on the back on medicine boxes as a P in a small box. Sales assistants in pharmacies can dispense such medicines with the correct training, but the pharmacist must be nearby to be able to intervene in the sale if necessary.

# Non-drug OTC products
In the US, medical devices may also be considered OTC.  For example, most diabetic management supplies are available OTC. The category of medical devices includes electronic blood glucose monitors, monitor test strips, and lancets. In some areas insulin syringes may also be available OTC; some areas have classified all syringes as limited OTC products as a measure of drug addiction harm reduction.

# Switches between prescription and OTC
As a general rule, over-the-counter drugs have to be primarily used to treat a condition that does not require the direct supervision of a doctor and must be proven to be reasonably safe and well-tolerated. OTC drugs are usually also required to have little or no abuse potential, although in some areas drugs such as codeine are available OTC (usually in strictly limited formulations or requiring paperwork or identification to be submitted during purchase). One of the oldest OTC drugs is aspirin.
Over time, drugs that prove themselves safe and appropriate for self-medication, may be switched from prescription to OTC. An example of this is diphenhydramine (Benadryl) which once required a prescription but now is available OTC nearly everywhere. Diphenhydramine is a deliriant, nevertheless, most recreational drug users find its effects uncomfortable rather than exciting. More recent examples are cimetidine and loratadine in the United States, and ibuprofen (Herron Blue/Nurofen) in Australia.
It is somewhat unusual for an OTC drug to be withdrawn from the market as a result of safety concerns, rather than market forces, though it does happen occasionally, phenylpropanolamine being one example.
Recently many U.S. drugstores have begun moving products containing pseudoephedrine into locations where customers must ask a pharmacist for them. A prescription is not required; the change is allegedly being made in an effort to reduce methamphetamine production.  Since the passage of the Methamphetamine Precursor Control Act, the purchase of pseudoephedrine in the United States is restricted and the identity of the purchaser is required to be obtained and recorded. In addition, pseudoephedrine itself is a mild stimulant- somewhere between caffeine and ephedrine. Nonetheless, these products are still considered OTC since no prescription is required.