Off-label use

Off-label use is the use of pharmaceutical drugs for an unapproved indication or in an unapproved age group, unapproved dosage, or unapproved form of administration. Both prescription drugs and over-the-counter drugs (OTCs) can be used in off-label ways, although most study of off-label use centers on prescription drugs. Off-label use is generally legal unless it violates specific ethical guidelines or safety regulations, but it does carry health risks and differences in legal liability.
Off-label use of medications is very common. Generic drugs generally have no sponsor as their indications and use expands, and incentives are limited to initiate new clinical trials to generate additional data for approval agencies to expand indications of proprietary drugs.
See also: Off-Label Drug Use: What You Need to Know at WebMD