The first-dose phenomenon is a sudden and severe fall in

blood pressure Blood pressure (BP) is the pressure of Circulatory system, circulating blood against the walls of blood vessels. Most of this pressure results from the heart pumping blood through the circulatory system. When used without qualification, the term ...

that can occur when changing from a lying to a standing position the first time that an

alpha blocker Alpha blockers, also known as α-blockers or α-adrenoreceptor antagonists, are a class of pharmacological agents that act as antagonists on α-adrenergic receptors ( α-adrenoceptors). Historically, alpha-blockers were used as a tool for p ...

drug is used or when resuming the drug after many months off. This

postural hypotension Orthostatic hypotension, also known as postural hypotension, is a medical condition wherein a person's blood pressure drops when they are standing up ( orthostasis) or sitting down. Primary orthostatic hypotension is also often referred to as ne ...

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 Prazosin, sold under the brand name Minipress among others, is a medication used to treat high blood pressure, symptoms of an enlarged prostate, and nightmares related to post-traumatic stress disorder (PTSD). It is an α1 blocker. It is a les ...

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

(Minipress) is most notorious for producing a first dose phenomenon. Other drugs of the same family,

doxazosin Doxazosin, sold under the brand name Cardura among others, is a medication used to treat symptoms of benign prostatic hyperplasia (enlarged prostate), hypertension (high blood pressure), and post-traumatic stress disorder (PTSD). For high bloo ...

(Cardura) and

terazosin Terazosin, sold under the brand name Hytrin among others, is a medication used to treat symptoms of an enlarged prostate and high blood pressure. For high blood pressure, it is a less preferred option. It is taken by mouth. Common side effec ...

(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 A diuretic () is any substance that promotes diuresis, the increased production of urine. This includes forced diuresis. A diuretic tablet is sometimes colloquially called a water tablet. There are several categories of diuretics. All diuretics in ...

), or were using

beta blockers Beta blockers, also spelled β-blockers, are a class of medications that are predominantly used to manage abnormal heart rhythms (arrhythmia), and to protect the heart from a second heart attack after a first heart attack (secondary prevention). ...

. Diuretics and beta blockers are frequently used to control

hypertension Hypertension, also known as high blood pressure, is a Chronic condition, long-term Disease, medical condition in which the blood pressure in the artery, arteries is persistently elevated. High blood pressure usually does not cause symptoms i ...

. For this reason, treatment with

(Minipress) should always be initiated with a low dose and should be taken at bedtime to avoid standing position.

Other drug classes with observed first dose hypotension

This effect is also observed after the administration of the first dose of drugs in the ACEi class ( angiotensin-converting enzyme inhibitor). This may occur with the class's better known side effect of dry cough (due to decreased breakdown of bradykinin), though there is no clear relationship between the two side effects. The first dose phenomenon in ACEi is reduced and made safer by avoiding diuretics for 24 hours prior to first dose, taking first dose at night (so avoiding falls, etc) and starting on low doses and titrating upwards.

References

{{reflist Clinical pharmacology

Other drug classes with observed first dose hypotension

See also

References