Caffeine is a mild diuretic, meaning it causes a person to urinate (pee) more. Drinking a moderate amount of caffeine isn't likely to cause dehydration, but it's probably a good idea to stay away from too much caffeine in hot weather, during long workouts, or in other situations where you might sweat a lot.
Caffeine also may cause the body to lose calcium, and that can lead to bone loss over time. Drinking caffeine-containing soft drinks and coffee instead of milk can have an even greater impact on bone density and the risk of developing osteoporosis.