I believe smaller doses can work and are effective, but only after you're ready to post on The Cured List.

Once you begin having more AF days than not the SEs will be revived (assuming you suffered them initially) if you find yourself in a drinking situation and abruptly take 50mg. This will lead to the temptation to skip the Nal. In this eventuality, take a 1/4 or 1/2 and you'll be all right. But daily drinking requires 50mg for the vast majority of us to cover all the receptors. Otherwise, little by little, mission creep seeps in.