Air travel: Can I take caffeine pills on a US national flight? UU.?

Caffeine is a legal medicine in the US UU And caffeine pills are sold without a prescription in every pharmacy and supermarket without a prescription (up to 200 mg per pill, I think).

Pills are generally allowed in checked or hand luggage. See https://www.tsa.gov/travel/security-screening/whatcanibring/all and search for "pills" or "medications." They do not fall into any prohibited category, such as sharp objects, explosives or liquids.