Naltrexone is a medication that is currently used to treat drug and alcohol addiction. Guanfacine is a medication that is currently used to manage the withdrawal symptoms in individuals undergoing opioid detoxification. A combination of these two medications may be beneficial in reducing heroin use in individuals addicted to heroin. This study will evaluate the effectiveness of naltrexone and guanfacine, alone and in combination, at reducing heroin use in heroin addicts.