Caffeine: general

Caffeine is a methylxanthine alkaloid, a central nervous system stimulant found naturally in coffee, tea, and other plants [1][2][3]. It's the most widely used psychoactive stimulant in the world [2], and it's also an approved drug: the FDA has approved it for apnea of prematurity, and it's used off-label for migraines, headaches after a spinal tap, and athletic performance [2][3].

For a healthy adult, a single dose up to about 200mg (roughly 3mg per kilogram of body weight) from all sources — coffee, tea, pills, energy drinks, food — doesn't raise safety concerns [4]. Spread across a day, total intake up to 400mg doesn't raise safety concerns for the general healthy adult population, with one exception: pregnant women, for whom this limit doesn't apply [4]. If you're using caffeine from more than one source in a day, add them up — they all count against that same daily total.

Caffeine's elimination half-life may range between 1.5 and 9.5 hours [5].

References

1. Caffeine | C8H10N4O2 | CID 2519 - PubChem — National Center for Biotechnology Information (NCBI)
2. Caffeine - StatPearls - NCBI Bookshelf — National Center for Biotechnology Information (NCBI)
3. Caffeine: Uses, Interactions, Mechanism of Action | DrugBank — DrugBank
4. Caffeine | EFSA — European Food Safety Authority
5. 2 Pharmacology of Caffeine — National Center for Biotechnology Information (NCBI)