I wouldn't really call it stomach pain that I feel - it is usually liver but occasionally kidneys or intestines. The pain is somewhat intense and lasts 5-25 seconds, 2-20 times daily. I don't even have to eat anthelmintics, just applying a wee bit of ivermectin or fenben cream to the rectum will give me 2-4 days of these pains.
My digestion is perfect and so is all of my blood work.
I assume pain is from the sharp-edges of the parasite forms moving through and scolex breaking free from organs. If this isn't actually the reason for the pain, I'm poisoning myself and will experience organ failure in the near future.
It seems very dangerous to me to get constipated on these meds, so take what you need to insure digestions OP.