Yes, you can use lactulose for your cat; it's an effective stool softener that's recommended by a lot of veterinarians. However, this needs a prescription from your vet. Its dosage is usually about 1/2 a teaspoon every 8 hours until your cat's stool becomes loose enough. One downside of this medication though that it's too sticky so you might face some difficulties in trying to make your cat intake it.