|MadSci Network: Neuroscience|
This is an excellent question, and I've been digging for the answer. Theobromine is not a controlled substance, and it is included in some weight-loss supplements. However, as you say, it is not sold in pure form like caffeine is.
Toxicity is not the reason. Theobromine is much less toxic than caffeine for humans. Pure caffeine is just as toxic to cats and dogs (or even slightly more so) than theobromine. (Incidentally, the reason for the difference in theobromine toxicity in humans and pets is that humans metabolize it much faster; the half-life in the blood of cats and dogs is about twice as long as in humans.) You can find lots of great data about these chemicals, including toxic doses in various species, by searching for each at this site: http://chem.sis.nlm.nih.gov/chemidplus/chemidheavy.jsp
My guess as to why theobromine is not sold is that it is much less potent than caffeine at producing responses. In fact, theobromine does not have a stimulant effect on the central nervous system like caffeine does. So for most people, theobromine tablets would not produce the effects they're looking for, or it would take about 10 times the amount to produce a response.
There may be a market for this, especially for people who have adverse reactions to caffeine, but not theobromine.
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