TLDR Using minoxidil on balding men's scalps can increase hair growth, according to a 12-month study.
In a 12-month study involving 56 patients with male pattern baldness, subjects were treated with either a 2% or 3% minoxidil solution or a placebo. The patients using minoxidil showed a significant increase in hair growth compared to the placebo group. The study found that minoxidil applied topically can partially reverse the balding phenomenon by decreasing vellus hair counts and increasing indeterminate and terminal hair counts. There was no significant difference between the effects of the 2% and 3% solutions.
138 citations,
August 1985 in “Journal of The American Academy of Dermatology” Minoxidil promotes hair growth in male pattern baldness.
55 citations,
February 1985 in “Archives of Dermatology” Using minoxidil on the scalp can help grow hair for people with hereditary baldness.
December 2017 in “Journal of Pigmentary Disorders” January 2006 in “The Chinese Journal of Clinical Pharmacology” 
11 citations,
April 1990 in “Australasian Journal of Dermatology” Minoxidil modestly increases hair growth in men with early baldness and is safe to use.
31 citations,
January 1989 in “Clinical and Experimental Dermatology” The study tested a hair treatment and found it can help grow hair, but won't stop baldness. It's safe and works well, but needs more testing.
8 citations,
May 1987 in “Journal of The American Academy of Dermatology” UV light does not significantly affect minoxidil's ability to treat male pattern baldness.
32 citations,
March 1987 in “Journal of The American Academy of Dermatology” Using minoxidil on balding men's scalps can increase hair growth, according to a 12-month study.
