Some people have a higher tolerance for caffeine. Though scientists can't seem to agree on whether caffeine can directly cause any health problems, there are several conditions have been consistently linked to caffeine consumption. For example, it can stimulate the production of gastric acid, leading to heartburn. It also increases calcium excretion, thus contributing to osteoporosis. However, two tablespoons of milk added to the coffee help offset this effect.
Caffeine is also a diuretic - it increases the loss of water.
Pregnant women should stay off caffeine. Studies have shown that pregnant women who consume 250mg to 300mg of caffeine per day have twice the risk of delivering underweight babies, who are then physically and developmentally at risk. People with hypertension, abnormal heart rhythms, ulcers and osteoporosis should also avoid caffeine. Parents should limit intake if their child is already hyperactive. But generally, if you have no medical reasons to avoid caffeine, then there's no cause for worry.