A generic drug is identical, or bioequivalent to a brand name drug in dosage form, safety, strength, route of administration, quality, performance characteristics and intended use.  Although generic drugs are chemically identical to their branded counterparts, they are typically sold at substantial discounts from the branded price.
 

Economics

The principal reason for the relatively low price of generic medicines is that these companies incur fewer costs in creating the generic drug, and are therefore able to maintain profitability while offering the drug at a lower cost to consumers. The costs of these generic drugs are so low that many developing countries can easily afford. For example Thailand is going to import millions of generic version pills of Plavix, a blood-thinning treatment to prevent heart attacks, costs just 3 US cents per pill from India, the leading manufacturer of generic drugs.

Generic manufacturers do not incur the cost of drug discovery, and instead are able to reverse-engineer known drug compounds to allow them to manufacture bioequivalent versions. Generic manufacturers also do not bear the burden of proving the safety and efficacy of the drugs through clinical trials, since these trials have already been conducted by the brand name company. In most countries, generic manufacturers must only prove that their preparation is bioequivalent to the existing drug in order to gain regulatory approval. It has been estimated that the average cost to brand-name drug companies of discovering and testing a new innovative drug (with a new chemical entity) may be as much as $800 million. However, these estimations are strongly disputed as much too high.

Generic drug companies may also receive the benefit of the previous marketing efforts of the brand-name drug company, including media advertising, presentations by drug representatives, and distribution of free samples. Many of the drugs introduced by generic manufacturers have already been on the market for a decade or more, and may already be well-known to patients and providers (although often under their branded name).

Prior to the expiration of a drug patent, a brand name company enjoys a period of "market exclusivity" or monopoly, in which the company is able to set the price of the drug at the level which maximizes profitability. This price often greatly exceeds the production costs of the drug, which can enable the drug company to make a significant profit on their investment in research and development. The advantage of generic drugs to consumers comes in the introduction of competition, which prevents any single company from dictating the overall market price of the drug. With multiple firms, the profit-maximizing price generally reflects the ongoing cost of producing the drug, which is usually much lower than the monopoly price.