The official numbers
The U.S. FDA sets 400 mg per day as the amount not generally associated with negative effects in healthy adults — that's about four 8 oz home-brewed cups or two grande Pike Places. For pregnancy, ACOG recommends capping at 200 mg per day. Adolescents should stay under 100 mg, and the American Academy of Pediatrics says caffeine has no place in young children's diets at all.
Single-dose guidance matters too: research on pure caffeine shows about 1.2 g is a documented toxic threshold and 10 g is a potential lethal dose. You cannot reach those numbers drinking coffee — you'd need physical volume first — but highly concentrated powders and pure liquids have sent people to emergency rooms. Energy drinks and shots deserve respect precisely because they compress doses.
What too much feels like, in order
Overdoing caffeine announces itself in stages: jitteriness and a racing heart first, then anxiety, upset stomach, and fragmented sleep. At high doses people report muscle twitches, confusion, and irregular heartbeat. If you feel fine on 600 mg daily, note that tolerance masks stimulation but doesn't change how long caffeine sits in your bloodstream at night — sleep studies still find the disruption.
Dependency is real but modest: typical withdrawal is a headache and fogginess for one to three days after stopping, manageable by stepping down 25% every few days.
Reading the limits in real drinks
The tracker on this site draws a hard line at your chosen daily cap and shows how each logged drink spends it. A few reference points from the database: a 20 oz Starbucks Venti Pike Place is 400 mg — the entire adult ceiling in one cup. A Bang or Reign is 300 mg. Dunkin's 24 oz iced medium is 297 mg. Meanwhile a 12 oz Coke spends 34 mg, and a green tea 28 mg — you can drink within your budget all day if you pick the low-caffeine lane.
Put it to work: cutoff calculator · daily tracker · 110-drink database