The organization that integrates scientific information for the UK National Health Service has recommended that the drug nalmefene can be prescribed to alcohol dependent patients for the purpose of reducing their drinking but not necessarily stopping it. Nalmefene, like the better known medication naltrexone, is an opiate antagonist that is intended to make the experience of drinking less rewarding. This reduction in subjective reward could make it more likely than a drinker will stop at 2, 3 or 4 drinks rather than going on to 5, 6, 7 or many more drinks.
Many people will find this concept strange, as they think that the purpose of opiate antagonists (indeed all alcohol-related medications) is to help problem drinkers continue pre-existent abstinence (e.g., Keep dry after leaving a detox center). Sometimes they do just that. However, from the point of view of behavioral extinction, one could argue that its actually a good thing when a person on an opiate antagonist decides to drink. They will then experience drinking behavior as less rewarding, which should over time reduce the behavior and also the craving to drink.
My colleagues and I, led by Dr. Natalya Maisel, conducted a meta-analysis of this question focused on the medication naltrexone, which we contrasted with a different medication prescribed to problem drinkers that is not an opiate antagonist (acamprosate). The entire research synthesis is available for free here, but the key finding for present purposes is that while acamprosate was more strongly associated with maintaining abstinence, naltrexone was better at limiting the number of drinks on drinking days.
Does this mean that every problem drinker can return to moderate drinking if they just take naltrexone or nalmefene? No. Problem drinkers who have more physical dependence symptoms (e.g., needing a drink in the morning, tremors, cravings), less social capital and worse mental health are generally unlikely to return to moderate drinking no matter what treatment they get. However, if dependence isn’t too advanced and the problem drinker has significant social and psychological resources available to support their efforts, they may indeed be able to go back to non-problem drinking, and medication can help with that. Even then, it’s a decision to make in consultation with a health professional, and the taking of the medication should be accompanied by additional counselling.