 There are a number of studies showing that caffeine has a short-term stimulatory effect on basal metabolic rate.However, in 1992, in a double-blind placebo controlled study, caffeine (at a dose of 200 mg daily) was found to be no more effective in promoting weight loss as compared to a placebo.Potential side effects of caffeine may include insomnia, anxiety, gastrointestinal discomfort, diarrhea, headaches and abnormal heart beat.