First dose phenomenon
Encyclopedia
The first dose phenomenon is a sudden and severe fall in blood pressure
that can occur when changing from a lying to a standing position the first time that an alpha blocker
drug is used or when resuming the drug after many months off. This postural hypotension usually happens shortly after the first dose is absorbed into the blood and can result in syncope
(fainting). Syncope occurs in approximately 1% of patients given an initial dose of 2 mg prazosin
or greater. This adverse effect is self-limiting and in most cases does not recur after the initial period of therapy or during subsequent dose titration.
The alpha blocker prazosin
(Minipress) is most notorious for producing a first dose phenomenon. Other drugs of the same family, doxazosin
(Cardura) and terazosin
(Hytrin), can also cause this phenomenon, though less frequently.
The cause is not clear. It occurs more commonly in patients who are salt and fluid volume depleted (as happens due to the use of diuretics), or were using beta blockers. Diuretics and beta blockers are frequently used to control hypertension
. For this reason, treatment with prazosin
(Minipress) should always be initiated with a low dose and should be taken at bedtime to avoid standing position.
Blood pressure
Blood pressure is the pressure exerted by circulating blood upon the walls of blood vessels, and is one of the principal vital signs. When used without further specification, "blood pressure" usually refers to the arterial pressure of the systemic circulation. During each heartbeat, BP varies...
that can occur when changing from a lying to a standing position the first time that an alpha blocker
Alpha blocker
Alpha-1 blockers constitute a variety of drugs which block α1-adrenergic receptors in arteries and smooth muscles.-Pharmacology:...
drug is used or when resuming the drug after many months off. This postural hypotension usually happens shortly after the first dose is absorbed into the blood and can result in syncope
Syncope (medicine)
Syncope , the medical term for fainting, is precisely defined as a transient loss of consciousness and postural tone characterized by rapid onset, short duration, and spontaneous recovery due to global cerebral hypoperfusion that most often results from hypotension.Many forms of syncope are...
(fainting). Syncope occurs in approximately 1% of patients given an initial dose of 2 mg prazosin
Prazosin
Prazosin, trade names Minipress, Vasoflex, Pressin and Hypovase, is a sympatholytic drug used to treat high blood pressure and Anxiety, PTSD and Panic Disorder. It belongs to the class of alpha-adrenergic blockers. Specifically, prazosin is selective for the alpha-1 receptors on vascular smooth...
or greater. This adverse effect is self-limiting and in most cases does not recur after the initial period of therapy or during subsequent dose titration.
The alpha blocker prazosin
Prazosin
Prazosin, trade names Minipress, Vasoflex, Pressin and Hypovase, is a sympatholytic drug used to treat high blood pressure and Anxiety, PTSD and Panic Disorder. It belongs to the class of alpha-adrenergic blockers. Specifically, prazosin is selective for the alpha-1 receptors on vascular smooth...
(Minipress) is most notorious for producing a first dose phenomenon. Other drugs of the same family, doxazosin
Doxazosin
Doxazosin mesylate, a quinazoline compound sold by Pfizer under the brand names Cardura and Carduran, is an α1a-selective alpha blocker used to treat high blood pressure and urinary retention associated with benign prostatic hyperplasia ....
(Cardura) and terazosin
Terazosin
Terazosin is a selective alpha 1 antagonist used for treatment of symptoms of an enlarged prostate...
(Hytrin), can also cause this phenomenon, though less frequently.
The cause is not clear. It occurs more commonly in patients who are salt and fluid volume depleted (as happens due to the use of diuretics), or were using beta blockers. Diuretics and beta blockers are frequently used to control hypertension
Hypertension
Hypertension or high blood pressure is a cardiac chronic medical condition in which the systemic arterial blood pressure is elevated. What that means is that the heart is having to work harder than it should to pump the blood around the body. Blood pressure involves two measurements, systolic and...
. For this reason, treatment with prazosin
Prazosin
Prazosin, trade names Minipress, Vasoflex, Pressin and Hypovase, is a sympatholytic drug used to treat high blood pressure and Anxiety, PTSD and Panic Disorder. It belongs to the class of alpha-adrenergic blockers. Specifically, prazosin is selective for the alpha-1 receptors on vascular smooth...
(Minipress) should always be initiated with a low dose and should be taken at bedtime to avoid standing position.