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What is Phenoxybenzamine and explain in detail?
Phenoxybenzamine (marketed under the trade names Dibenzyline and Dibenyline) is a non-selective, irreversible alpha blocker.

Uses
It is used in the treatment of hypertension, and specifically that caused by pheochromocytoma. It has a slower onset and a longer-lasting effect compared with other alpha blockers.
It was also the first alpha blocker to be used for treatment of benign prostatic hyperplasia, although it is currently seldom used for that indication due to unfavourable side effects.
It has been used in the treatment of hypoplastic left heart syndrome.It is also used in complex regional pain syndrome (CRPS) type 1 due to its antiadrenergic effects. It has shown to be beneficial if used in the first 3 months of the CRPS diagnosis.

Investigational
Phenoxybenzamine has long been known to block ejaculation without affecting semen quality or ability to achieve orgasm, which could make it an effective male contraceptive. This effect is completely reversible, and is believed to be the result of alpha-1 adrenoceptor blockade in the longitudinal muscles of the vas deferens. A dose of 20 mg/day results in aspermia due to reversible paralyzing effects on the vas deferens, ampulla, and ejaculatory ducts. Due to these actions, phenoxybenzamine is also useful for the treatment of premature ejaculation in men.

Pharmacology
Phenoxybenzamine is used as an anti-hypertensive due to its efficacy in reducing the vasoconstriction caused by epinephrine (adrenaline) and norepinephrine. Phenoxybenzamine forms a permanent covalent bond with adrenergic receptors.  Based on known information about the structures of these receptors, it likely involves attack by the cysteine at position 3.36 in transmembrane helix 3 to form a stable linkage. Thus, it remains permanently bound to the receptor, preventing adrenaline and noradrenaline from binding