Once a day topical use of Rogaine (topical minoxidil 2% and 5%) seems to be almost as effective as using it twice a day. The reason is, that although minoxidil has a relatively short half-life of several hours when given orally, when topically applied, it has a half-life of 22 hours in the skin. This suggests that once a day dosing is a reasonable option. It is important to realize that Pfizer, the company that now makes Rogaine, specifically states that it will be less effective if used only once a day.
