Most drugs have one intended use. They were researched and tested with that one use in mind. Studies on animals and then trials with humans are done before a drug is released to the public.

Sometimes, just by chance, a drug will have an effect on something else. For example, mustard gas was used in previous wars to kill people. This led to its later use as an agent to kill cancer cells.

Likewise, pregnant women used Thalidomide for nausea in the early 1960s. It resulted in birth defects and was taken off the market. Today, it has been found effective in treating some forms of cancer. This is an off-label use until studies are done just on its use with cancer.

One final example that has been in the news lately is Provigil. This is a stimulant like caffeine and amphetamines. It's FDA-approved for only one condition: narcolepsy. Narcolepsy is a condition of daytime sleepiness. The person falls asleep suddenly at any time of the day. Use of Provigil for people who want to stay awake all night is considered off-label and isn't approved by the FDA. After careful study of safety issues, some drugs are approved for off-label uses.