Side effects and uses
What are the side effects?
Side effects these drugs are related to their mechanism, particularly to their vasodilatory effects. The two important side effects of these drugs are
- Postural hypotension
- Reflex tachycardia
- Nasal congestion
- Sexual dysfunction
As these drugs act as direct vasodilators decreasing the vascular tone, they may cause sudden fall in the blood pressure with change in the posture. This may lead to falling of the patients on standing. Due to long duration of action, this effect is more pronounced with phenxoybenzamine.
Our physiological system has highly active reflex mechanisms which are activated under certain conditions to bring the situation again to normal state marinating homeostasis. Maintenance of blood pressure is such an active mechanism and any raise or drop in the blood pressure is immediately corrected by these reflex mechanisms.
Non-selective alpha blockers act on alpha1 receptors and produce vasodilatation decreasing blood pressure. This activates baroreceptor reflex which increases sympathetic discharge from the CNS. These released catechol amines act on beta1 receptors on the heart increasing rate of contraction leading to tachycardia.
These drugs can also produce tachycardia in another way. They block alpha2 receptors and as discussed above this increases release of norepinephrine from nerve terminals which act on beta1 receptors to produce tachycardia.
These drugs can also produce cranial vasodilatation leading to headache.
This is one more unwanted effect due to nasal vasodilatation. These drugs increase congestion due to block of nasal air pathways due to dilated capillaries. This may result in sneezing, difficulty in breathing and nasal block.
Note: alpha agonist like phenylephrine and oxymetazoline are used as nasal decongestants to relive nasal block.
One of the important side effect of concern on long term use of alpha blockers is sexual dysfunction in males. Erection of penis is a vasoconstrictory response where blood vessels of erectile tissue are constricted. Quite oppositely, ejaculation is a vasodilatory response terminating the erection. Since alpha blockers produce vasodilatation they increase ejaculation leading sexual problems like dysfunctional ejaculation or difficulty to maintain erection in males.