According to Mayo Clinic, adults should try to stay under 400 milligrams of caffeine consumption per day, while adolescents should drink no more than 100 milligrams of caffeine in a day or avoid it altogether. Excessive caffeine intake can lead to digestive problems, increased heart rate, irritability and muscle trembling.
Mayo Clinic explains that caffeine tolerance varies in each person. Some people are able to handle more than four cups of coffee per day, while others can barely tolerate one cup. It is important for people to gauge their body's reaction to caffeine and avoid drinking so much that it begins to cause side effects. People who suffer with anxiety and insomnia are especially advised to refrain from eating or drinking things with caffeine in them.