Hello,
The diarrhea has a cause. Until you find out what that is you are simply trying to resolve a clinical sign.
If your dog isn’t on a heartworm prevention that treats for intestinal parasites you should ask your vet to look for these. If your pet is young and unvaccinated I worry about parvovirus. If they are old I worry about disease and deficiency. Your dogs weigh tells me nothing about what the cause might be. Please call a vet and get better help to resolve this. Chronic diarrhea can cause dehydration, emaciation and even death.