There are two reasons. First, anytime caffeine is added to a flavor it can alter the taste significantly, so we have to be very selective when determining which flavors can contain caffeine. At this time Hammer Nutrition formulates two caffeinated flavors of Hammer Gel - Espresso is a natural in terms of a flavor that can and should contain caffeine (50 mg/serving), and the tart flavor of Tropical makes it suitable to contain a small amount of caffeine (25 mg/serving).
Secondly, the addition of caffeine to any product will make it more acidic. Though the higher level acidity is minor, it may increase the potential for stomach irritation in some people, especially when consuming large quantities of caffeine/caffeinated products.
