Both caffeine and alcohol have physiological and psychological effects upon the body. While caffeine is a well-known stimulant that individuals often use to increase feelings of alertness and attention, alcohol is an inhibition-lowering drug and a relaxant. Perhaps counterintuitively, given the different ways the compounds affect mental state, they both increase blood pressure. While the exact mechanism whereby each chemical affects blood pressure is still under investigation, researchers have examined several possibilities.
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Both alcohol and caffeine are common recreational drugs used for various purposes, some of which are social and some of which have to do with altering mental status. Patients with high blood pressure, however, are generally counseled to avoid both alcohol and caffeine, on the grounds that they can increase blood pressure. The American Stroke Association notes that most patients appear to experience increased blood pressure with caffeine use, while the American Academy of Family Practitioners suggests that individuals with high blood pressure limit alcohol consumption.
The mechanisms by which alcohol and caffeine increase blood pressure still aren't well understood. Research published in the "Journal of Applied Physiology" in 1998 suggests that caffeine's effects on blood pressure may have to do with ways in which the compound alters chemical and hormonal activity in the body. The University of Maryland Medical Center suggests that those who drink frequently may have a more difficult time controlling blood pressure, and that this effect may have to do with alcohol inhibiting the effects of pressure-reducing medications.
In a 2008 article published on the website Science Daily, reporters note that a recent study conducted in England suggests that one mechanism for why alcohol affects blood pressure--particularly in certain individuals--might be genetic. The body converts alcohol to a very toxic substance called acetaldehyde, then converts and excretes acetaldehyde. Some individuals lack the genes for acetaldehyde-converting enzymes, meaning that they have strong reactions to alcohol, which include flushing and nausea. The study found that these individuals also appear to experience much higher blood pressure after alcohol consumption.
Both caffeine and alcohol are well known to be diuretics, meaning that they cause the body to expel more water via the urinary system. Since this decreases total blood volume and, in extreme cases, can result in dehydration, it's not uncommon to postulate that alcohol and caffeine would lower, rather than raise, blood pressure. Typically, notes Dr. Lauralee Sherwood in her book, "Human Physiology," diuretics decrease blood pressure. Alcohol and caffeine are exceptions to this rule.
While the mechanisms whereby alcohol and caffeine increase blood pressure may not be well known, the fact that an increase occurs is recognized in the medical community. In the case of caffeine, notes the American Stroke Association, individuals using caffeine-containing beverages routinely experience an increase in blood pressure by approximately 10 mmHg. The association notes that this effect is potentially damaging to those with already high blood pressure, and recommends that elderly patients and those with heart disease avoid caffeine.