There are several possible reasons why people can report stomach aches when
taking lactase. First and foremost is that lactase is not taken unless food
containing milk products are also eaten in the meal. This leaves open the
possibility that the real culprit is the milk. It may be that rather than
lactase being the problem, taking too little lactase is. Or that the real
ailment is a milk hypersensitivity that lactase won't help. Or that a
different food is the problem.

But a direct cause and effect between lactase and stomach aches is hard to
substantiate. There are no reports of this in the medical literature.
(Doctors love to get  publishing credits by coming up with just these types
of associations, so there is great pressure to find these and get a report
known.) And it is unclear exactly how lactase, a natural enzyme that passes
through the intestines without absorption, would trigger these complaints in
the first place. It is just barely possible that some people might be
reacting to the mannitol that is in some lactase pills, although the quantity
is so small that this is also unlikely.

I hate doubting people who report sincere problems, but pinpointing the exact
cause of a problem, itself vaguely reported, relating in some way to
something taken in by mouth is one of the hardest associations for anybody to
prove. I'm a strong proponent of the "if it makes you feel better, then do
it" school, though, so if you feel uncomfortable taking any pill, or any
particular brand of any pill (and there are often huge variations among
brands, then don't. I hope it works for you.

Steve Carper
Steve Carper's Lactose Intolerance Clearinghouse
http://ourworld.compuserve.com/homepages/stevecarper