Caffeine Lowers Cerebral Blood Flow

Nov 21, 2015
The effect of daily caffeine use on cerebral blood flow: How much caffeine can we tolerate?

Caffeine is a commonly used neurostimulant that also produces cerebral vasoconstriction by antagonizing adenosine receptors. Chronic caffeine use results in an adaptation of the vascular adenosine receptor system presumably to compensate for the vasoconstrictive effects of caffeine. We investigated the effects of caffeine on cerebral blood flow (CBF) in increasing levels of chronic caffeine use. Low (mean = 45 mg/day), moderate (mean = 405 mg/day), and high (mean = 950 mg/day) caffeine users underwent quantitative perfusion magnetic resonance imaging on four separate occasions: twice in a caffeine abstinent state (abstained state) and twice in a caffeinated state following their normal caffeine use (native state). In each state, there were two drug conditions: participants received either caffeine (250 mg) or placebo. Gray matter CBF was tested with repeated‐measures analysis of variance using caffeine use as a between‐subjects factor, and correlational analyses were conducted between CBF and caffeine use. Caffeine reduced CBF by an average of 27% across both caffeine states. In the abstained placebo condition, moderate and high users had similarly greater CBF than low users; but in the native placebo condition, the high users had a trend towards less CBF than the low and moderate users. Our results suggest a limited ability of the cerebrovascular adenosine system to compensate for high amounts of daily caffeine use.


Jun 23, 2015
I believe it, I've long thought that I'm dumber when extremely caffeinated lol
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Jan 26, 2020
The brain has a parallel architecture. Reduced cortical blood flow is a selective process for parallel programming of the brain by the autonomic system by favoring emergence of metabolically active networks


Apr 8, 2020
Caffeine Constrict Blood Vessels

Caffeine can affect the body and brain in a number of ways. While it is most commonly used for alertness and mental focus, caffeine consumption does have other possible side effects that should not be overlooked. Blood vessel constriction, also known as vasoconstriction, is one of these potential side effects.

Caffeine's Effects on the Body

Caffeine, a substance found naturally in tea leaves, kola nuts, cocoa beans and coffee, quickly passes into the brain once ingested. It then stays in the body for many hours, which means its effects can last for hours. Among the potential side effects of caffeine include increased heart rate, anxiety, difficulty sleeping, nausea, restlessness and frequent urination. Suddenly stopping caffeine use can cause withdrawal symptoms, including drowsiness, headaches and irritability.

Causes of Vasoconstriction

Vasoconstriction is the narrowing of blood vessels. When vasoconstriction occurs, blood flow is slowed down or partially blocked. It can occur in response to psychological conditions or drugs, such as decongestants, pseudoephedrine or caffeine. According to MedlinePlus, medications exist to both increase and reduce vasoconstriction, depending in the condition being treated.

Caffeine, Vasoconstriction and the Brain

A study published in 2009 in "Human Brain Mapping" set out to determine the effect of caffeine on cerebral blood flow. The researchers found that caffeine use reduced cerebral blood flow by an average of 27 percent. This means that, while caffeine is known for improving mental energy, it can actually reduce the total amount of blood in the brain, which could potentially result in reduced cognitive function.

Limiting Caffeine Intake

By limiting your caffeine intake, you may be able to avoid vasoconstriction and decreased blood flow in the brain. The study published in "Human Brain Mapping" found that those who consumed high levels of caffeine had less cerebral blood flow, when compared to low and moderate caffeine users. In the study, 45 milligrams per day was considered low dosage, 405 milligrams per day was considered moderate and 950 milligrams per day was considered a high level of caffeine.


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Emily Wax, RD, CDN, University of Virginia Health System, Charlottesville, VA. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.

The effect of daily caffeine use on cerebral blood flow: How much caffeine can we tolerate?


this is a pretty serious side effect from caffeine.
when you are hypothyroid and have a slow metabolic rate, than your bloodflow is already slow. Taking caffeine makes things only worse.
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Apr 18, 2018
"The bad effects ascribed to coffee usually involve administering large doses in a short period of time. While caffeine is commonly said to raise blood pressure, this effect is slight, and may not occur during the normal use of coffee. Experimenters typically ignore essential factors. Drinking plain water can cause an extreme rise in blood pressure, especially in old people, and eating a meal (containing carbohydrate) lowers blood pressure. The increased metabolic rate caffeine produces increases the cellular consumption of glucose, so experiments that study the effects of coffee taken on an empty stomach are measuring the effects of increased temperature and metabolic rate, combined with increased adrenaline (resulting from the decrease of glucose), and so confuse the issue of caffeine’s intrinsic effects.

In one study (Krasil’nikov, 1975), the drugs were introduced directly into the carotid artery to study the effects on the blood vessels in the brain. Caffeine increased the blood volume in the brain, while decreasing the resistance of the vessels, and this effect is what would be expected from its stimulation of brain metabolism and the consequent increase in carbon dioxide, which dilates blood vessels.

In the whole body, increased carbon dioxide also decreases vascular resistance, and this allows circulation to increase, while the heart’s work is decreased, relative to the amount of blood pumped. But when the whole body’s metabolism is increased, adequate nutrition is crucial."

Ray Peat.​


Oct 11, 2016
I believe it, I've long thought that I'm dumber when extremely caffeinated lol

It is pretty obvious to me as well. If I have like 5 coffees without much food, then I can get a nose bleed - when it rarely happens it can be delayed for up to a day, though.
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