Prazosin, trade names Minipress,Vasoflex and Hypovase, is a sympatholytic drug used to treat high blood pressure (hypertension). It belongs to the class of alpha-adrenergic blockers, which lower blood pressure by relaxing blood vessels. Specifically, prazosin is selective for the alpha-1 receptors on vascular smooth muscle. These receptors are responsible for the vasoconstrictive action of norepinephrine, which would normally raises blood pressure. By blocking these receptors, prazosin reduces blood pressure.
Prazosin is orally active and has a minimal effect on cardiac function due to its alpha-1 receptor selectivity. Unlike other less selective alpha blockers that also block presynaptic alpha-2 receptors, it doesn't prevent inhibition of norepinephrine release. Uninhibited norepinephrine release usually causes an increased reflex tachycardia through the sympathetic baroreflex response that, in turn, increases cardiac output. However, when blood pressure drops, the body does not increase cardiac output to compensate for the loss in pressure when a person takes prazosin. The antihypertensive characteristics of prazosin make it a second-line choice for the treatment of high blood pressure.
Prazosin is also useful in treating urinary hesitancy associated with prostatic hyperplasia by blocking alpha-1 receptors, which control constriction of both the prostate and ureters. Although not a first line choice for either hypertension or prostatic hyperplasia, it is an excellent choice for patients who present with both problems concomitatntly.
This medication has shown to be effective in treating severe nightmares in children, associated with PTSD symptoms. Also veterans have been treated successfully at the Oregon VA for sleep disturbance related to PTSD. Doses are lower for this purpose than for control of blood pressure.
Side effects of prazosin include orthostatic hypotension, syncope, and nasal congestion. The orthostatic hypotension and syncope are associated with the body's poor ability to control blood pressure without active alpha-adrenergic receptors. Patients on prazosin should be told not to stand up too quickly, since their poor baroreflex may cause them to faint as all their blood rushes to their feet. The nasal congestion is due to dilation of vessels in the nasal mucosa.
One phenomenon associated with prazosin is known as the "first dose response", in which the side effects of the drug, especially orthostatic hypotension, are especially pronounced after the first dose.
Another common side effect of prazosin (and doxazosin) is priapism.
Since 1983 prazosin has revolutionized the management of severe scorpion stings.