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

blood pressure Blood pressure (BP) is the pressure of 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 "blood pressure" r ...

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

alpha blocker Alpha (uppercase , lowercase ; grc, ἄλφα, ''álpha'', or ell, άλφα, álfa) is the first letter of the Greek alphabet. In the system of Greek numerals, it has a value of one. Alpha is derived from the Phoenician letter aleph , which ...

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 standing up or sitting down. Primary orthostatic hypertension is also often referred to as neurogenic orthostatic hyp ...

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 is an α1 blocker medication primarily used to treat high blood pressure, symptoms of an enlarged prostate, and nightmares related to post-traumatic stress disorder (PTSD). It is a less preferred treatment of high blood pressure. Other ...

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 names Cardura among others, is a medication used to treat symptoms of benign prostatic hyperplasia (enlarged prostate) and hypertension (high blood pressure). For high blood pressure, it is a less preferred option. ...

(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 Hypertension (HTN or HT), also known as high blood pressure (HBP), is a long-term medical con ...

(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 cardiac arrhythmia, abnormal heart rhythms, and to protect the heart from a second myocardial infarction, heart attack after a first heart ...

. Diuretics and beta blockers are frequently used to control

hypertension Hypertension (HTN or HT), also known as high blood pressure (HBP), is a long-term medical condition in which the blood pressure in the arteries is persistently elevated. High blood pressure usually does not cause symptoms. Long-term high bl ...

. 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 Angiotensin-converting-enzyme inhibitors (ACE inhibitors) are a class of medication used primarily for the treatment of high blood pressure and heart failure. They work by causing relaxation of blood vessels as well as a decrease in blood volum ...

). 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