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Chapter: Modern Medical Toxicology: Cardiovascular Poisons: Diurets, Antihypertensives and Antiarrhythmics

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Minoxidil - Sympatholytic Drug Cardiovascular Poison

Minoxidil is an efficacious antihypertensive in patients with drug-resistant, severe hypertension.

Minoxidil

Minoxidil is an efficacious antihypertensive in patients with drug-resistant, severe hypertension. It acts through an active metabolite minoxidil N-O sulfate, which produces arteri-olar vasodilatation (without effect on capacitance vessels). Minoxidil increases blood flow to skin, skeletal muscle, GI tract, and heart. Apart from its use in hypertension, minoxidil is also used topically to promote hair growth in patients with male pattern baldness.

Toxicokinetics

About 95% bioavailability; protein binding is not significant; volume of distribution is 2 to 3 L/kg; elimination half-life varies from 2.3 to 28.9 hours.

Adverse Effects

·      Pericardial effusion, pulmonary hypertension, dermatitis, breast tenderness or gynaecomastia, oedema, pulse and BP changes, shortness of breath, dizziness, headache, redness of conjunctivae, and increased growth of facial, and (later) body hair. Some of these effects appear even on topical application.

·              Several cases of allergic contact dermatitis have been reported when minoxidil has been used in the treatment of baldness.

·       There have also been several case reports of women applying 5% topical minoxidil, over a period of 2 to 3 months, to treat androgenetic alopecia and subsequently developing severe hypertrichosis of the face and extremities. The excessive hair growth on the face and limbs gradually disappeared over a period of several months after discontinuation of minoxidil.

Clinical (Toxic) Features

·      Moderate to severe hypotension.

·      Angina pectoris and haemorrhagic pericarditis have also been reported after minoxidil overdose.

·      Coma can occur.

Treatment

In treating hypotension due to minoxidil, avoid cardiac stimu-lating sympathomimetics such as adrenaline and noradrena-line. When a vital organ is underperfused, use phenylephrine, vasopressin, dopamine, or angiotensin II. Haemodialysis may be useful in minoxidil overdose.

 

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