Let me reiterate IF THIS IS AN EMERGENCY, PLEASE CONTACT YOUR VETERINARIAN IMMEDIATELY. . That being said, lasix is a diuretic. It causes a loss of fluid by increasing urination. This happens by changing the electrolyte balance in your dog. How much danger your puppy is in depends on how many of the lasix your puppy ate and how long ago. Seek immediate veterinary assistance. If the puppy just ate the lasix, getting the stomach pumped will help. If it has been a while, the puppy may need to be on IV fluids.