Alcohol dependence is a chronic disease, which can develop when alcohol is heavily used over longer periods of time. Alcohol affects various brain regions, including the opioid receptor system, which mediates euphoric and pleasurable effects of alcohol. By blocking alcohol effects at these receptors, the opioid antagonists naltrexone and nalmefene can reduce alcohol "liking" and "craving" and thus support alcohol dependent patients in cutting down their drinking. 50 studies with 7793 participants were included in the review, in most studies treatment was provided over a period of three months. The review shows that more patients who took naltrexone were able to reduce the amount and frequency of drinking than those who took an identical appearing, but inert substance (placebo). On average, one out of nine patients was helped by naltrexone. Naltrexone does not have serious side effects, but gastrointestinal symptoms like nausea, stomach pain and loss of appetite are common. Some patients also get tired from naltrexone. For injectable formulations of naltrexone, which can be advantageous for patients who have problems with taking their medication on schedule, and the second opioid antagonist nalmefene, the database is still too sparse to allow final conclusions. Nevertheless, the available studies indicate that these drugs might have comparable effects on drinking than oral naltrexone has. Naltrexone does not cause dependency and unlike disulfiram, another medicine that is sometimes used to treat alcohol dependence, it does not make patients feel sick if they drink alcohol while taking it.