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<SNIP debate on what we mean by intelligence and whether IQ is valid>
On a more down to earth level...
Caffeine helps alertness (no sh*t) and in moderate amounts, there doesn't seem to be much evidence for toxicity. When I seriously need it, I take it in pill form (available in any drug store) so as to avoid all other crap that's in coffee and soft drinks. I sometimes take ephedrine (minithins, available in many convenience stores and truck stops) to increase alertness, improve mood, and overcome procrastination. I've observed that after taking a lot of ephedrine I get very irritable as the mood elevation wears off. For a while I did ritalin (prescription drug for treating hyperactivity/ADD) and to me it felt more or less like ephedrine, but not as strong.
When I do this, I'm probably harming my health. It's a tradeoff. I get what I need done right now, and promise myself not to get into these "due yesterday" situations in the future. Obviously I don't pop pills every day.
This is probably not an intelligence boost as you define it, but the question is, what is it that you're planning to use your intelligence for? I use mine to: 1) solve computer problems and 2) cram large numbers of gerontology papers into my brain. My main problem is being scared of the magnitude of work in front of me and going into denial. Chemically enhanced alertness and mood help prevent this.
All the above is my individual, anecdotal, subjective experience. It might all be placebo effect, and may affect you in a different way if at all. YMMV
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