Minoxidil: Mechanisms of Action on Hair Growth

A.G. Messenger; J. Rundegren


The British Journal of Dermatology. 2004;150(2) 

In This Article

Summary and Introduction

We have known for over 30 years that minoxidil stimulates hair growth, yet our understanding of its mechanism of action on the hair follicle is very limited. In animal studies, topical minoxidil shortens telogen, causing premature entry of resting hair follicles into anagen, and it probably has a similar action in humans. Minoxidil may also cause prolongation of anagen and increases hair follicle size. Orally administered minoxidil lowers blood pressure by relaxing vascular smooth muscle through the action of its sulphated metabolite, minoxidil sulphate, as an opener of sarcolemmal KATP channels. There is some evidence that the stimulatory effect of minoxidil on hair growth is also due to the opening of potassium channels by minoxidil sulphate, but this idea has been difficult to prove and to date there has been no clear demonstration that KATP channels are expressed in the hair follicle. A number of in vitro effects of minoxidil have been described in monocultures of various skin and hair follicle cell types including stimulation of cell proliferation, inhibition of collagen synthesis, and stimulation of vascular endothelial growth factor and prostaglandin synthesis. Some or all of these effects may be relevant to hair growth, but the application of results obtained in cell culture studies to the complex biology of the hair follicle is uncertain. In this article we review the current state of knowledge on the mode of action of minoxidil on hair growth and indicate lines of future research.

Minoxidil was introduced in the early 1970s as a treatment for hypertension. Hypertrichosis was a common side-effect in those taking minoxidil tablets[1,2] and included the regrowth of hair in male balding.[3] This led to the development of a topical formulation of minoxidil for the treatment of androgenetic alopecia in men and subsequently in women. The 2% product was first marketed for hair regrowth in men in 1986 in the United States and the 5% product became available in 1993.

Despite much research over 20 years we still have only a limited understanding of how minoxidil stimulates hair growth. Nevertheless, understanding minoxidil's mechanism of action is important, both from the point of view of developing more effective treatments for hair loss disorders and for the insights it may give into the biology of hair growth. In this article we review what is known about the pharmacology of minoxidil, with particular reference to its action on hair growth, and suggest directions for future research.