The principal alkaloid of the Cacao bean is theobromine, a close structural relative of caffeine. Theobromine differs from caffeine by only one methyl group and is a metabolite of caffeine (that means you can find this stuff in your liver after you drink a cup of coffee). It is one of the molecules that makes chocolate taste characteristically bitter.
Cacao contains between 1.5-3% theobromine and 0.2-0.4% caffeine (note that there is a small amount of caffeine in dark chocolate). Theobromine has a ten-fold less stimulating effect to humans than caffeine does so you won't get a "espresso rush" eating chocolate, although lots of sugar will provide a bit of oomph . For dogs however chocolate is a "no-no", 100-200 mg of theobromine per kg of a dogs body weight can cause cardiac and central nervous system distress since they cannot metabolize (i.e. get rid of it) as easily as humans can.
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