http://www.walgreens.com/library/ask/aapById.jsp?aap...

However, every medication is subject to a limited patent life. The traditional patent protection timespan is usually 17 years. In reality though, the patent period is much shorter (approximately 7 years) due to the extended length of time required to bring new medication to market. After the patent expires, other drug companies can produce and sell the generic equivalent of the brand name medication once they receive FDA approval.