A common side effect of excessive caffeine consumption is increased heart rate or tachycardia. High doses of caffeine can lead to nervousness, restlessness, and anxiety, negatively impacting mood and cognitive function. In some individuals, it may also cause sleep disturbances, leading to fatigue and decreased performance in the long term. Caffeine can also result in digestive issues or headaches in sensitive individuals. It's essential to moderate caffeine intake to avoid these side effects while still benefiting from its potential performance-enhancing effects when consumed in appropriate amounts.