Bristol University researchers found that drinkers develop a tolerance to both the anxiety-producing and the stimulating effects of caffeine, meaning that it only brings them back to baseline levels of alertness, not above them.

Although frequent consumers feel alerted by caffeine, especially by their morning tea, coffee, or other caffeine-containing drink, evidence suggests that this is actually merely the reversal of the fatiguing effects of acute caffeine withdrawal, wrote the scientists, led by Peter Rogers of Bristol's department of experimental psychology.

The team asked 379 adults -- half of them non/low caffeine consumers and the other half medium/high caffeine consumers -- to give up caffeine for 16 hours, and then gave them either caffeine or a dummy pill known as a placebo.

Participants rated their levels of anxiety, alertness and headache. The medium/high caffeine consumers who got the placebo reported a decrease in alertness and increased headache, neither of which were reported by those who received caffeine.

But measurements showed that their post-caffeine levels of alertness were actually no higher than the non/low consumers who received a placebo, suggesting caffeine only brings coffee drinkers back up to normal.

The researchers also found that people who have a genetic predisposition to anxiety do not tend to avoid coffee.

In fact, people in the study with a gene variant associated with anxiety tended to consume slightly larger amounts of coffee than those without it, Rogers wrote in a study in the Neuropsychopharmacology journal, published by Nature.

This suggests that a mild increase in anxiety may be a part of the pleasant buzz caused by caffeine, he said.

Medicine Coffee