Caffeine effects differ from person to person. Several general effects are stomachache, headaches, and insomnia. Caffeine has numerous effects, but it is okay in moderation. The health effects of caffeine have been widely studied. Caffeine competitively inhibits dissimilar adenosine receptors, thereby producing mild central nervous system stimulant effects. Short term side effects such as headache, nausea, and anxiety have been exposed as symptoms of mild caffeine consumption.