Under U.S. Food and Drug Administration
regulations intended to ensure the safety and effectiveness of pharmaceutical medicine, drug manufacturers may only label and market prescription medications
with reference to "approved" uses. That is to say, doses and routes of administration that have through clinical trials
been proven to meet accepted standards of both safety and efficacy
in treating a given condition.
However, this does not mean that these are the only manners in which such drugs may be used. Once a drug is given approval to go to market, doctors may freely prescribe its use in any manner and for any purpose they see fit. Usages other than those a drug has been officially approved and labeled for are known as "off-label" uses.
Doctors prescribing a medication on an off-label basis may simply have a hunch that it will have a positive effect, on the basis of the molecular composition of the drug and the biological structures involved (there is, after all, a reason why pre-meds slog through so much organic chemistry). However, many off-label uses are ones for which there is a significant volume of supporting literature, but for which official certification has not yet been received.
Uses which start off as off-label may later gain official approval. For example, the drug bupropion hydrochloride, initially approved for the treatment of depression, was later tested and approved as a smoking cessation aid. As a marketing decision, it is sold under a different name for each purpose (Wellbutrin and Zyban, respectively), though this is not, either legally or pharmaceutically speaking, necessary. Even before Zyban was released, while it was still in testing, doctors who had heard of its potential through medical journals, conferences, or word of mouth could and did prescribe Wellbutrin to smokers.
This dual certification, however, might be more an anomaly than the norm. The sort of clinical testing that the FDA demands for certification can cost hundreds of millions of dollars. In this case, GlaxoWellcome (now GlaxoSmithKline) expected that it would recoup this investment from the increased profits to be gained by marketing the drug to a significant market of nicotine addicts.
Those conditions which only affect a small number of potential customers, however, might not constitute a market large enough to make certification a profitable proposal. Off-label use allows patients with such "orphan conditions" to reap a medication's benefits nonetheless. Likewise, if a new use for a drug is discovered after a substantial number of years have passed since its introduction, it is possible that no company will want to invest in official testing if they will have few or no years left to recoup the cost by leveraging the exclusive production rights granted by patent.