Coffee is a mild stimulant and apparently is not harmful if used sparingly. Most people can tolerate and enjoy a moderate amount (three cups a day or less) with no harmful effects. Recent studies have shown that moderate coffee drinking is unlikely to cause adverse effects on the average person's health and can even be mildly beneficial.
However, some people find that even a small amount causes nervousness, and they would probably be better off not to drink it at all. Neither should a person let coffee drinking become so necessary that the day is ruined without it. The apostle Paul advises against even such "moderate" addictions or overindulgence in I Corinthians 6:12-13: "All things are lawful for me, but all things are not helpful. All things are lawful for me, but I will not be brought under the power of any. Foods for the stomach and the stomach for foods, but God will destroy both it and them."
The same principle applies to tea. Caffeinated soft drinks pose additional and often more significant risks because of the high sugar or artificial sweetener content.