Hypertrichosis (excess hair growth)
Hypertrichosis is hair growth beyond what is normal for a person's age, sex or ethnicity, or for that part of the body, and it is a recognised side effect of minoxidil.
Full definition
Hypertrichosis means extra hair growth that is abnormal for a person's age, sex or ethnicity, or for a particular area of the body (Wendelin and colleagues, Journal of the American Academy of Dermatology, 2003). It can be present from birth or develop later, and it can affect the whole body or just one area. Several medicines can cause it, including minoxidil, ciclosporin and phenytoin. In fact, according to the British Association of Dermatologists, minoxidil was first prescribed as a tablet for high blood pressure, and the increase in facial hair seen in women taking it is what led to the development of topical minoxidil for hair loss. The Loniten SmPC, written for its licensed use in severe high blood pressure, says hypertrichosis "occurs in most patients treated with minoxidil" and that patients should be warned before starting. It describes lengthening, thickening and darkening of fine body hair, usually appearing 3 to 6 weeks after starting and first on the face, and says it usually reverses one to six months after stopping. Those figures come from blood pressure treatment, so they should not be read as the rate with the lower doses used off-label for hair loss. Repeated application of minoxidil to the skin can also cause hair growth in a limited area. If you are taking oral minoxidil or using topical minoxidil and notice unwanted hair, tell your prescriber, who can review your treatment. Our pages on oral minoxidil and facial hair and oral vs topical minoxidil cover the options. Unwanted hair can also be removed in the usual ways, though it will regrow while the cause continues.
What it does not mean
Hypertrichosis is not the same as hirsutism. Hirsutism is thick, dark hair in a male pattern in women, on areas such as the face, chest or back, and the NHS says it is usually linked to changes in androgen ("male" hormone) levels. Minoxidil-related hair growth is a medicine side effect, and it usually settles after stopping. New excess hair in a woman who is not taking minoxidil or another known cause should be checked by a GP.
UK regulatory source
Loniten 2.5 mg SmPC sections 4.1 and 4.4 (eMC); British Association of Dermatologists leaflet, Hair loss: male pattern, June 2024; NHS, Excessive hair growth (hirsutism); Wendelin DS et al. Hypertrichosis. J Am Acad Dermatol 2003 (PMID 12582385); DermNet, Hypertrichosis