How long should minoxidil be left on the scalp?

Minoxidil: 4 Hours Needed (8-12 hours better!)

Minoxidil needs to be left on the scalp at least 4 hours and 8 hours (or longer) is much better. 

This recommendation I give comes from a study done in 1990. In this particular research study, twenty-two male study participants completed a four-way, multiple-dose, randomized crossover study to investigate  the relationship between how long the drug was applied to the scalp and the amount of minoxidil absorption. A 2% topical solution was used in these studies.  The researchers compared how much minoxidil was absorbed at 1 hr, 4 hr compared to 11.5 hours. 

What were the results?

Relative to the amount absorbed after a contact time of 11.5 h, absorption was approximately 50% complete by 1 h and greater than 75% complete by 4 h.

Conclusion

Although minoxidil should ideally be left on 8 hours, a significang amount of benefit can still be achieved with 4 hour use. Even at 1 hour about one half is absorbed but clearly this is less than ideal. Understanding the absorption of minoxidil is important to help improve the ability of patients to 'stick with' the treatment. 

Reference

Ferry JJ et al. Relationship between contact time of applied dose and percutaneous absorption of minoxidil from a topical solution. J Pharm Sci 1990 Jun;79(6):483-6