I searched alpha blockers and got this from drugdigest.org.
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Alpha Blockers
Alpha blockers are commonly used to treat hypertension (high blood pressure) or benign prostatic hyperplasia (BPH). This class of drugs is generally not preferred as a first line treatment to lower blood pressure, but is most frequently used in combination with other medications when blood pressure is difficult to control.
Alpha blockers may be a good choice of drug therapy in men who have both hypertension and benign prostatic hyperplasia (BPH), a common disorder in older men that involves enlargement of the prostate and can cause urinary problems such as difficulty urinating and increased frequency of urination.
Drugs in this Class
Doxazosin (Cardura)
Prazosin (Minipress)
Terazosin (Hytrin)
Summarizing the Evidence
There have only been a few studies directly comparing these drugs with one another for the treatment of high blood pressure. Based on those studies, all three drugs lowered blood pressure similarly and there does not seem to be a major advantage of one alpha blocker over another in terms of blood pressure lowering. In addition, there was not a significant difference in side effects.
Based on the results of a large study known as the Antihypertensive and Lipid-Lowering Treatment to Prevent Heart Attack Trial (ALLHAT), alpha blockers should generally not be used as first line therapy for the treatment of hypertension, unless they are being used for both hypertension and benign prostatic hyperplasia (BPH).
Alpha blockers are generally well tolerated with mild side effects such as drowsiness, dizziness, lightheadedness, heart palpitation, lightheadedness, and fainting. There may be more dizziness associated with alpha blockers especially after the first few doses, as compared to other classes of drugs used to lower blood pressure. These side effects can sometimes be minimized or avoided by taking the alpha blocker at bedtime and starting at low doses and gradually working up to higher doses slowly if needed. No major differences in the frequency of side effects among the alpha blockers are apparent.
Dosing and Administration
Doxazosin and terazosin are typically taken once daily. However, prazosin is typically taken three times per day because it has a shorter duration of action.
Generic Availability
Doxazosin (Cardura), prazosin (Minipress) and terazosin (Hytrin) are all available in generic formulations.
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Trying to find the category for clonidine is frustrating. It is variously relegated to "other hypertensive medication", "centrally acting hypertensive medication", "alpha-2 adrenergic receptor" and "alpha adrenergic agonist". A couple other drugs in this class are guanabenz and methyldopa.
I found this information on clonidine, but I can't remember where:
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Clonidine is a centrally-acting alpha-2 agonist. It selectively stimulates receptors in the brain that monitor catecholamine levels in the blood. These receptors close a feedback loop that begins with descending sympathetic nerves from the brain that control the production of catacholamines (epinephrine , also known as adrenaline, and norepinephrine) in the adrenal medulla. By fooling the brain into believing that catacholamine levels are higher than they really are, clonidine causes the brain to reduce its signals to the adrenal medulla, which in turn lowers catacholamine production and blood levels. The result is a lowered heart rate and blood pressure, with side effects of dry mouth and fatigue.
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