Though theobromine is not as powerful as caffeine in its effects. Yet, its effects do mimic that of caffeine in that it acts like a stimulant as it increases heart rate. There has also been a study published by the University of Utah in 1993 that shows theobromine may increase the change of older men developing prostate cancer.
Furthermore, Theobromine can cause sleeplessness/restlessness, tremors, anxiety, as well as contribute to increased production of urine Additional side effects include loss of appetite, nausea, and vomiting.
However, you would have to eat a ridiculously large amount of chocolate to for the above to apply. For example, the lowest published toxic dose (TDLo) for Theobromine is 26 mg/Kg –for a human that is, in animals it is much lower.
Therefore, if you are human you can eat large quantities of chocolate safely. However, I would advise you should not eat more chocolate than your total body weight or else you may be in trouble.