Heroin addiction is a serious health problem with no available medical treatment for preventing relapse. Naltrexone is a medication that is currently used to treat substance addiction. It acts by blocking the "high" feeling produced by drugs and alcohol. Guanfacine, an antihypertensive medication, is currently used to manage the withdrawal symptoms in individuals undergoing opioid detoxification. While each of these medications is useful in the treatment of heroin addiction, a combination of the two drugs may be more effective than either medication alone. The purpose of this study is to compare the effectiveness of naltrexone, guanfacine, and a combination of naltrexone and guanfacine at reducing drug relapse in heroin addicts.