Over-the-counter (OTC) drugs are medicines that may be sold directly to a consumer without a prescription from a health care professional, as compared to prescription drugs, which may only be sold to consumers possessing a valid prescription. In many countries, OTC drugs are selected by a regulatory agency to ensure that they are ingredients that are safe and effective when used without a physician's care. OTC drugs are usually regulated by active pharmaceutical ingredients (APIs), not final products. By regulating APIs instead of specific drug formulations, governments allow manufacturers freedom to formulate ingredients, or combinations of ingredients, into proprietary mixtures. 
The term over-the-counter may be somewhat counter-intuitive, since, in many countries, these drugs are often located on the shelves of stores like any other packaged product. In contrast, prescription drugs are almost always literally passed over a counter from the pharmacist to the customer. Some drugs may be legally classified as over-the-counter (i.e. no prescription is required), but may only be dispensed by a pharmacy employee after an assessment of the patient's needs and/or the provision of patient education. In many countries, a number of OTC drugs are available in establishments without a pharmacy, such as general stores, supermarkets, gas stations, etc. Regulations detailing the establishments where drugs may be sold, who is authorized to dispense them, and whether a prescription is required 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" 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 deal with the vast number 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 after review by expert panels. This meant that certain classes of OTC drugs were not required to obtain an NDA and could remain on the market if they conformed to the monograph guidelines for doses, labeling, and warnings.
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. Examples of OTC substances approved 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.
The FDA requires that OTC products are labeled with an approved Drug Facts label to educate consumers about their medications. These labels comply to a standard format and are intended to be easy for typical consumers to understand. Drug Facts labels include information on the product's active ingredient(s), indications and purpose, safety warnings, directions for use, and inactive ingredients.
 Restricted OTC Substances
An ill-defined third category of substances are those products having over-the-counter status from the FDA, while being simultaneously subject to other restrictions on sale. While these products are legally classified as OTC drugs, they are typically stored behind the pharmacy counter and are only sold in stores employing a registered pharmacist; such items may be unavailable in convenience or grocery stores that stock other non-restricted OTC medications.
For example, many U.S. drugstores have moved products containing pseudoephedrine, an OTC product, into locations where customers must ask a pharmacist for them. A prescription is not required; the change has been made in an effort to reduce methamphetamine production. Since the passage of the Illinois Methamphetamine Precursor Control Act and the subsequent Federal Combat Methamphetamine Epidemic Act of 2005, the purchase of pseudoephedrine in the United States is restricted. Sellers of pseudoephedrine must obtain and record the identity of the purchaser and enforce quantity restrictions. Some states may have more stringent requirements (e.g. Oregon, where a medical prescription is required to purchase any quantity of pseudoephedrine). Despite these restrictions, products containing the substance are still considered OTC in all states except Oregon, since no prescription is required.
A similar regulation applies to various forms of Emergency Contraception. The FDA considers these products to be OTC substances for women aged 17 or over, but prescription drugs for younger women.  To enforce this restriction and to provide counseling and education on proper use, an agreement between the manufacturer and the FDA requires that these drugs are stored behind the pharmacy counter. Women may obtain the medication without a prescription after providing proof of age to pharmacy staff and receiving any necessary patient education.
Furthermore, some Schedule V controlled substances may be classified as OTC products in certain states. Such drugs are sold without a prescription, but are subject to record-keeping rules, quantity and/or age restrictions, and must be dispensed by a pharmacy.
 United Kingdom
In the United Kingdom, medication will fall into one of three categories
- Prescription Only Medication (POM), which is usually only available with a valid prescription from a doctor or via internet suppliers. A pharmacist has to be on the premises for POM medicines to be dispensed, required by law. The medicine has been specifically prescribed for the patient holding the prescription, so it is considered safe for only the recipient to take. Just a small example of these include most Antibiotics, and allAntidepressants or Antidiabetic medications. Drugs also included as POM are high strength painkillers such as Oxycodone and Tramadol as well as medications such as Sildenafil (Viagra), Diazepam (Valium) as well as certain topical preparations such as Corticosteroids. These medicines are often sold by drug dealers, especially those marked as "CD POM" which are controlled due to abuse risk such as Diconal, Temazepam and Methadone.
- Pharmacy Only Medication (P), containing chemicals, strengths or sizes which are more likely to cause interactions or need basic instruction on use by a health professional. The pharmacist will, in theory, determine whether the medicine is safe for the particular customer based on their responses to a set of questions; many pharmacies sell P medicines "no questions asked" instead. Sometimes this is known as "behind-the-counter" medication. These do not require a doctors prescription, just a discussion with a pharmacist. Examples of these include some sleep aid tablets such as Nytol, Human de-worming tablets such as Mebendazole, painkillers with small amounts of Codeine (up to 12.8mg per tablet), and pseudoephedrine.
- General Sales List (GSL), available off the shelf with no pharmacy training required to sell (so they can be sold anywhere, such as supermarkets). Generally considered safe for most people when taken correctly. Examples of these include 16-packs (or less) of painkillers such as Paracetamol & Nurofen as well as a host of other safe medications such as small pack sizes of Antiallergy tablets, Laxative medication and skin creams.
Medication available only with a prescription is marked somewhere on the box/container with [POM]. Pharmacy only products are marked with [P]. A prescription is not required for [P] medicines and pharmacy sales assistants are required by RPSGB codes to ask certain questions, which varies for what the customer says. If they ask for a specific product the Pharmacy Assistant must ask Who is it for, How long have you had the symptoms, Are you allergic to any medication, Are you taking any medication (WHAM Questions). If a customer asks for a remedy i.e. hayfever then the 2WHAM questions muct be followed Who is it for, What are the symptoms, How long have you had the symptoms, Have you taken any action towards your symptoms and Are you taking any other medication. It is with this information that the pharmacist can halt the sale, if need be. No [POM] [P] or [GSL] products that are stocked in a pharmacy can be sold or dispensed or pre made prescriptions being collected until a pharmacist is signed in as a responsiable pharmacist and is on the premises. Some medication available in supermarkets and petrol stations is sold there only in smaller packet sizes. Often, larger packs will be marked as [P] and available only from a pharmacy. Frequently, customers buying larger than usual doses of [P] medicines (such as DXM, promethazine, codeine or Gee's linctus) will be queried, due to the possibility of abuse. 
 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, often 3-5 years, drugs that prove themselves safe and appropriate as prescription medicines, 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 an anti-histamine. 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 is one such example, after it was removed from sale in the United States over concern regarding strokes in young women.
In the United Kingdom, it was announced In February 2007, that Boots the Chemist would try over the counter sales of Viagra in stores in Manchester, England (previous available as prescription only). Men aged between 30 and 65 would be eligible to buy four tablets after a consultation with a pharmacist..
 See also
 External links
-  FREE software to better understand OTC drug and drug interaction. Consumer can freely download software in smart phone or use web widget to check drug-drug interaction.;
-  The Tan Sheet is the weekly source of OTC industry news in the US;
-  OTC Product News comments on news and events effecting the OTC industry;
- Over-the-counter (OTC) Medicines at FamilyDoctor.org, maintained by the American Academy of Family Physicians. Contains extensive information on over-the counter drugs and their responsible use, including specific guidance on several drug classes in question-and-answer format and information on common drug interactions.