there are several studies that show expired medications on average only loose 5% of the effectiveness.
check this one: http://www.redorbit.com/news/health/771253...ates/index.html
"The FDA Shelf Life Extension Program has tested hundreds of drugs for the U.S. military since 1985 and found that, on average, they were good for 5 years after expiration. The program saved $296 million on drug replacements in 2005 alone, the FDA said. "