Minoxidil is a vasodilator and was exclusively used as an oral drug (LonitenÂ®) to treat high blood pressure. It was, however, discovered to have the interesting side-effect of hair growth and reversing baldness, and in the 1980s, Upjohn Corporation produced a topical solution that contained 2% minoxidil to be used to treat baldness and hair loss, under the brand name Rogaine in the United States, and Regaine outside the United States. Treatments usually include a 5% concentration solutions that are designed for men, whereas the 2% concentration solutions are designed for women. It is unknown how the drug stimulates hair growth.
In 2007 a novel, foam-based formulation of 5% Minoxidil was shown to be an effective treatment of androgenetic alopecia without the usual side-effects of the topical solution. 
The patent on minoxidil expired on February 13, 1996.
As a drug to combat hair loss, the most common side effect is itchy scalp. In some cases minoxidil may initially cause an increase in hair loss.
There have been cases of allergic reactions to minoxidil or the non-active ingredient propylene glycol, which is found in some forms of the topical version, such as Rogaine. Large amounts of minoxidil can cause hypotension, and it has been found that using petroleum jelly or tretinoin on the scalp with minoxidil can cause too much of the drug absorption by the scalp, as can using the drug on sunburned scalps.
If a person uses minoxidil to stop hair loss for a length of time and then stops taking the drug, hair loss will occur again.
Other side-effects may include:
- acne on the area where it is being used as a topical solution
- headaches and/or lightheadedness
- very low blood pressure
- irregular or fast heart beat
- blurred vision
- chest pain
All the side-effects in the above list except for acne may be an indicator that too much of the drug is being used.
It has also been found that the drug can be passed from a mother to a child via breast milk.
Minoxidil for Trans men
One of the side effects of testosterone therapy for trans men is an increased susceptibility to male pattern baldness (MPB). Using finasteride (Propecia) to slow or reverse hair loss can adversely affect facial hair growth as well so minoxidil is often used instead until sufficient facial hair growth has been achieved. RogaineÂ® (Minoxidil â€“ available without prescription) is sold as 2% and 5% solutions. The 5% solution is not recommended for use by women because it may cause the adverse effect of unwanted facial hair growth in a small minority of patients. However this hair is finer and may not resemble normal male facial hair, so it may not be advantageous to trans men desiring a beard. Minoxidil may cause skin irritation and itching. 1 cc is applied twice daily to the scalp (predominantly in the areas where hair loss is greatest.) It may take several months to show effects and may cause a slight paradoxical worsening of hair loss initially (which does eventually recover.)
Minoxidil for Trans women
Minoxidil may be used to treat hair loss in trans women. It may even be more effective in trans women taking female hormones because hair regrowth would not be opposed by testosterone and its derivatives, such as dihydrotestosterone (DHT). Since minoxidil is a vasodilator which has the potential of decreasing blood pressure there may be a reaction in those who use spironolactone, another drug that lowers blood pressure, which could result in blood pressure falling below safe levels. Since hormone levels in trans women taking HRT fall in the female range, the 2% minoxidil solution would probably be sufficient and the 5% version would be, at best, a waste of money.
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