Athletes take caffeine to increase energy levels, boost strength and power and speed metabolism to promote weight loss. It is among the most popular performance enhancing drugs in the world. A study conducted by Gordon Warren in the Medicine Science Sports Exercise, found that caffeine improves muscle endurance and strength. Most studies found that caffeine increased performance in team sport movements, power-base sports and weight training. Side effects include anxiety, hyperactivity, cardiac arrhythmias and insomnia. Many bodybuilders find that consuming small amounts of caffeine before workouts increase training intensity.