The initial design of minoxidil (also called 2,4-pyrimidinediamine, 6-(1-piperidinyl)-, 3-oxide) was as a potent peripheral vasodilator agent for the treatment of severe refractory hypertension in the 1970s. Due to its serious side effects, oral minoxidil was reserved for cases of severe hypertension that were reluctant to maximum doses of three antihypertensive agents.
Additionally, about one-fifth of patients under oral minoxidil treatment developed hypertrichosis. In 1987, a topical form was developed for the treatment of androgenic alopecia, initially for males and subsequently also for females. This article focuses specifically on topical minoxidil, the most commonly used form of the molecule.
Topical minoxidil is available in two forms: a solution (liquid form) and a foam. The solution contains alcohol and propylene glycol, two molecules necessary to dissolve minoxidil and increase its uptake in the tissues. Formulations containing 2% and 5% minoxidil are generally used in scalp alopecia in patients who are over 18 years of age. Clinicians can use minoxidil in children; however, it is considered off-label use. Long-term use of minoxidil is necessary to maintain the clinical results, as these effects regress with drug discontinuation.
Current uses of topical minoxidil include:
Topical minoxidil (empirical formula C9H15N5O) is a hair growth stimulator. Its mechanism of action is not well-established. Scalp sulfotransferase changes minoxidil into minoxidil sulfate, the active form of the molecule. Variations between individuals in sulfotransferase activity may be the cause of the discrepancy in minoxidil efficiency.
Minoxidil acts by shortening the telogen phase and thus causing the quiescent hair follicles to enter prematurely into the anagen phase. The shortening of the telogen phase may induce telogen effluvium after the initiation of minoxidil therapy. Also, minoxidil extends the duration of the anagen phase. Lastly, increased hair length and diameter is a clinical effect of minoxidil.
The initial effects of minoxidil occur after approximately eight weeks of treatment, and maximal effects take place after four months.
Minoxidil appears to act on the potassium channels of vascular smooth muscles and hair follicles, which may induce the following effects:
Minoxidil has demonstrated to possess anti-fibrotic properties secondary to its effect on collagen synthesis.
Minoxidil is available in the United States as an over-the-counter topical agent. Dosing is twice daily (1 ml each dosage). No scalp massage is necessary after use. Minoxidil uptake is about 50% after an hour and 75% after 4 hours. Some practitioners associate micro-needling with topical minoxidil to enhance its efficiency, but more studies are needed to assess the value of this potential relationship.
While oral minoxidil is not FDA-approved for hair loss, clinical trials have displayed effectiveness using oral minoxidil at various doses (0.25 to 2.5 mg daily).
Studies have shown that 5% minoxidil is more effective than 2% minoxidil in the treatment of alopecia. Clinical response to minoxidil is more pronounced if the onset of alopecia is within five years (mainly in young adults), and the hair follicles are not deeply miniaturized.
Minoxidil is generally well tolerated. However, some adverse effects reported by patients include:
Minoxidil is contraindicated in patients who have a history of hypersensitivity to the drug or its components (such as propylene glycol).
The use of minoxidil is not advised in pregnant and breastfeeding women. While minoxidil is not known to be teratogenic, reports exist of rare cases of congenital disabilities.
Patients using topical minoxidil require regular monitoring for scalp changes and localized/generalized hypertrichosis. Hypotension is rarely reported in patients using topical minoxidil.
Percutaneous toxicity is uncommon after the conventional use of minoxidil. There is no known antidote for minoxidil toxicity after massive oral ingestion. Accidental oral ingestion of minoxidil can result in vomiting and does rarely require hospitalization. There are reported cases of hypotension, tachycardia, and/or electrocardiogram (ECG) changes after accidental ingestion. Intravenous fluids and vasopressor agents may manage refractory hypotension. Gastric wash and activated charcoal may be necessary to prevent systemic toxicity in massive accidental ingestions of minoxidil.
While there are several medications produced by the pharmaceutical industry to manage male pattern baldness, effectiveness between individuals varies. Minoxidil has been present for three decades and is widely prescribed by nurse practitioners, physician assistants, primary care providers, and dermatologists. Minoxidil is considered a first-line treatment to aid hair growth, but it shows inconsistent efficacy. Some patients see a vast improvement, while others notice minimal changes. Healthcare workers should educate patients that each individual may react to minoxidil differently, and hair growth occurs best with consistent medication adherence.
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