Caffeine Myths That Won't Go Away

Caffeine does increase urine output slightly, but research consistently shows that the fluid you take in from a caffeinated beverage more than offsets what you lose. A 2003 review published in the Journal of Human Nutrition and Dietetics found no significant difference in hydration status between moderate caffeine consumers and non-consumers. For most healthy adults, caffeinated drinks count toward fluid intake. Very high doses may tip the balance, but typical coffee or tea consumption does not. For more on separating hydration fact from fiction, see hydration myths that keep people drinking the wrong amount.

There's an important distinction here between physical dependence and addiction. Regular caffeine use can lead to tolerance (needing more for the same effect) and withdrawal symptoms — headaches, fatigue, irritability — if you stop suddenly. These are signs of physical dependence. However, addiction involves compulsive use despite serious social, health, or occupational harm. The American Psychiatric Association's DSM-5 recognizes caffeine withdrawal as a condition but does not classify caffeine use disorder as a formal substance use disorder in the same category as alcohol or opioids. Dependence is real; addiction in the clinical sense is a different bar.

This myth likely grew from early concerns about caffeine's effect on calcium absorption. While caffeine does cause a small, transient reduction in calcium absorption, the effect is minor and easily offset by adequate dietary calcium — a glass of milk, for instance, more than compensates. Large-scale studies tracking children's growth have not found caffeine to be a driver of reduced height. That said, high caffeine intake is generally not recommended for children and adolescents for other reasons, including effects on sleep quality and anxiety. But stunted growth is not among the documented concerns.

Caffeine temporarily raises blood pressure and heart rate, which is why this myth took hold. However, numerous large observational studies — including research drawing on hundreds of thousands of participants — have found no significant link between moderate caffeine intake (generally defined as up to about 400 mg per day, roughly three to four standard cups of coffee) and cardiovascular disease in healthy individuals. Some research even suggests potential protective associations. The picture is different for people with certain arrhythmias or other heart conditions, who should follow their doctor's specific guidance.

Variants in the CYP1A2 gene affect how quickly the liver breaks down caffeine. Fast metabolizers clear it from their system rapidly, meaning an evening espresso has little effect on their sleep. Slow metabolizers may still feel the effects of an afternoon cup hours later. Tolerance built up through regular use also plays a role. So the person who insists they can drink coffee at 9 p.m. and fall asleep easily isn't necessarily making excuses — their biology may genuinely handle caffeine differently than yours.