Meds containing lactose can cause GI problems in patients with lactose intolerance.
It's possible...but not very common.
Most lactose intolerant people can consume up to two cups of milk per day without experiencing symptoms. A person would have to be exquisitely sensitive to have symptoms with the small amounts of lactose present in some medications.
For example, a one cup serving of milk contains 11 GRAMS of lactose...but most patients consume 500 mg or less of lactose from their daily medication regimen.