Common side effects of Lactulose in cats and dogs include diarrhea, vomiting, flatulence, and abdominal pain. It could also cause drooling in cats. If your pet produces more than three soft stools in a day, consider it as an alarm and immediately contact your vet. Lactulose can cause facial swelling, cold limbs, hives, and seizures in pets who are prone to allergies. It can alter insulin requirements in diabetic cats and dogs. Also, it could cause drug interactions with other laxatives, antacids, warfarin, neomycin, and gentamicin.