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:

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.