Opioid antagonists are a type of drug which blunts the effects of narcotics such as heroin and morphine, and might help reduce nicotine addiction by blocking some of the rewarding effects of smoking. Our review identified eight trials of naltrexone, a long-acting opioid antagonist. The trials included over 1200 smokers. Half the trials gave everyone nicotine replacement therapy and tested whether naltrexone had any additional benefit. Compared to a placebo, naltrexone did not increase the proportion of people who had stopped smoking, at the end of treatment, or at six months or more after treatment, either on its own or added to NRT. The available evidence does not suggest that opioid antagonists such as naltrexone assist smoking cessation.