What it does

Caffeine is the world's most-used stimulant. It blocks adenosine (the drowsiness signal) so alertness, heart rate, and blood pressure rise. Effects begin in 15–45 minutes and last several hours.

Safer-use principles

• —Most healthy adults tolerate up to ~400 mg/day; more can trigger anxiety, palpitations, and insomnia.
• —Stop caffeine 6–8 hours before sleep. Poor sleep is where most of caffeine's harm lives.
• —Dehydration and empty stomachs sharpen the jitters. Water and food help.
• —Combining with other stimulants (nicotine, energy-drink stacks, ADHD meds) multiplies cardiac load.

Dangerous combinations

Caffeine masks the sedating effects of alcohol without reducing impairment — you feel awake but are still drunk. Very high doses with stimulant medication can cause dangerous heart rhythms.

If something goes wrong

Severe overdose is rare but possible with concentrated caffeine powders/pills. Chest pain, vomiting, confusion, or seizures — call 911.