Call the nearest ER vet. Lasix over doses can cause serious issues.
What happens if I overdose the pet: Seek emergency veterinary medical attention if you believe you have given your pet too much medication. Symptoms of furosemide overdose may include loss of appetite, weakness, dizziness, confusion, fainting.
http://www.1800petmeds.com/Furosemide-prod10083.html
It can cause seizures, electrolyte imbalances and irregular heart beat.