According to an article in the british medical journal ,nalmefene in alcohol misuse is bad medicine. I haven't got access but that's the replies to the article, by doctors:
There's only medium evidence for nalmefene use, and the results speak of an average reduction of 2 days of heavy drinking per month and average reduction of 1 drink per day. Not that big.
It's recommended for people who don't manage to cut down after two weeks of psycho-social intervention, who also drink more than 7.5 units per day (that's very roughly half a liter of 14% wine per day) but who also drink less than the amount needed for medically supervised detox.
http://www.bmj.com/content/348/bmj.g1531/rapid-responses
Another place is a the most recent review of alcohol dependency drugs:
http://www.ncbi.nlm.nih.gov/pubmed/24825644
There's only medium evidence for nalmefene use, and the results speak of an average reduction of 2 days of heavy drinking per month and average reduction of 1 drink per day. Not that big.