Consuming 750 mg of caffeine daily in three doses increases cortisol levels throughout the day when individuals are exposed to mental stress, physical exercise, or eating after exercise, and this increase occurs similarly in both men and women, though the overall stress response size may differ by sex.

Consuming caffeine before a mentally stressful task leads to a 15–20% higher increase in cortisol levels compared to taking a placebo, and this effect is similar in men and women.

Taking 300 or 600 milligrams of caffeine daily for five days lowers the cortisol spike that occurs when you later take another 250 milligrams of caffeine, but cortisol levels still rise significantly, especially after afternoon doses, showing that the body does not fully adjust to caffeine over this period.

Taking 600 mg of caffeine per day for five days leads to a smaller increase in cortisol levels after a later caffeine dose compared to taking 300 mg per day, suggesting the body's stress hormone system adjusts more strongly at higher doses.

Consuming caffeine on the same day as a stress test does not change the level of cortisol released in response to stress in healthy young adults. The pattern of cortisol response is more likely related to regular, long-term caffeine consumption than to a single dose.

Even after taking 600 mg of caffeine daily for five days, the body still shows a measurable increase in cortisol levels after a subsequent dose, indicating that the stress response to caffeine does not fully disappear with short-term daily use.