Minoxodil, a vasodilator, C 9 H 15 N 5 O, administered orally for the treatment of hypertension marketed as Lotinen  was first developed in the 1970's as a blood pressure medication  because of its ability to cause blood vessels to dilate. However, some patients began noticing an interesting side effect: their facial hair began to thicken, and in some cases thickening was noted in previously balding scalp tissue.

Investigators made note of this observation and began to experiment with topical application of liquid solutions containing minoxidil. Studies demonstrated that a topical solution applied to the scalp thickened thinning hair in a statistically significant percentage of subjects. 

Previously approved by the FDA as a vasodilator in the treatment of refractory hypertension, the 2% solution was first marketed for hair regrowth in men in 1986 in the United States and the 5% product subsequently became available in 1993.  Upjohn Corporation obtained a patent for the formula; the company marketed their minoxidil solution as Rogaine™ in the U.S. and Canada, and as Regaine™ in elsewhere in the world.

The mechanism by which minoxidil promotes hair growth is not fully understood. Minoxidil is a potassium channel opener, causing hyperpolarization of cell membranes. Minoxidil is less effective in cases where there is a widespread scalp hair loss. In addition, its effectiveness has largely been demonstrated in younger men (18 to 41 years of age). Minoxidil use is not indicated for fronto-temporal hair loss, but central (vertex), or top of head, balding only.

Minoxidil is a vasodilator. Here it is hypothesized that by widening blood vessels and opening potassium channels, the drug allows more oxygen, blood and nutrients to the hair follicle. This can also cause follicles in the telogen phase to shed, usually soon to be replaced by new, thicker hairs (in a new anagen phase).

In 1997, a foam-based version of minoxidil was formulated as an alternative to the liquid solution. Although generally considered safe, as with any potent drug side effects may occur and they include burning or irritation of the eye; itching; redness or irritation at the treated area; unwanted hair growth elsewhere on the body.  

Somewhat more rare but significantly more severe side effects include severe allergic reactions (rash; hives; itching; difficulty breathing: tightness in the chest; swelling of the mouth, face, lips, or tongue); chest pain; dizziness; fainting; fast heartbeat; sudden, unexplained weight gain; swollen hands or feet.