How much caffeine does it take to produce an irregular heart-beat or other caffeine related illnesses?

Variable: This is highly variable depending on the individual and problems them may have as well as medications they may be taking. Check out caffeineweb.Com for excellent discussions on this.

It depends: Drinking coffee can actually help some relax, particularly if they are in some sort of withdrawal from caffeine which is a real issue for some. So individual variables are such that hard and fast rules about dosing could be hard to establish. Generally if someone is drinking less than 2 cups in a day the side effects are minor. If sensitive, even a half cup late in the day may be problematic.