Minoxidil was first introduced as an antihypertensive medication and the discovery of its common adverse event, hypertrichosis, led to the development of a topical formulation for promoting hair growth. To date, topical minoxidil is the mainstay ...
www.ncbi.nlm.nih.gov
"Topical minoxidil has been considered safe; however, some patients experienced side effects after application. The most common side effect of MS is irritant contact dermatitis with the typical symptoms of itching and scaling. The incidence is lower with 2% MS than it is with 5% MS. Allergic contact dermatitis can also occur because of PG or minoxidil itself. A patch test should be performed to determine the causative agents. Although an allergic reaction to minoxidil could occur, it is infrequent. If patients are allergic to PG, the vehicles should be substituted with butylene glycol, glycerin, or polysorbate. Moreover, MF, a PG-free formula, should be prescribed instead and if the reactions persist, then allergy to minoxidil should be suspected. In this scenario, the use of all minoxidil preparations should be discontinued.
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Oral minoxidil is mostly metabolized via the liver by conjugation with glucuronic acid. The metabolized minoxidil is excreted through the kidneys 3–4 hrs after administration, but the vasodilation property may persist for up to 72 hrs.
1 Serious adverse effects including sodium and fluid retention, and cardiovascular effects (eg ischemic heart disease, pericardial effusion, and pulmonary hypertension) have been reported with systemically administered minoxidil.
12 Sodium and fluid retention causes a rise in body weight and in severe cases, congestive heart failure occurred. This is due to the redistribution of blood flow from the outer to the inner cortex of the kidney as well as plasma-renin activity.
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2 Ischemic heart disease was reported, which might have been due to higher oxygen demand as heart rate and cardiac output increased.
4 Pericardial effusion caused by minoxidil occurs around 5% of patients with unknown mechanism.
12 Pulmonary hypertension due to increased pulmonary artery pressure and high cardiac output from minoxidil has also been described.
12 Other adverse effects include hypertrichosis, occasional pulsating headache, itchy eyes, skin rashes such as bullous eruptions, and polymenorrhea.
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90 In our opinion, the benefits of the oral medication are not sufficient to justify exposure to the risk of potential side effects."
Keep in mind this is just one article I found. I can post more if you want. They're all relevant as well (this one was from 2019).
