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I can certainly sympathize with you as far as the cost of things. Pet care is not inexpensive and with 8 cats it can really add up I'm sure.
As far as your cat, I have to say that I firmly believe she needs at least a thorough physical exam, especially if she is losing weight and not grooming well. Your friend is correct in that Metronidazole is a medication that is used to treat cats with colitis which is what it sounds like your cat is experiencing, but it is not the appropriate treatment for all cats with this condition.
Colitis is an inflammation of the large bowel. The signs can be very intermittent. The signs with this condition can vary in severity, from normal stools to mushy to very soft, all even within one day at times. Seeing mucus or blood is not unusual as well. Many pets with colitis have GI discomfort and some will even vomit. It is not common however for a cat to lose weight from this problem.
Colitis can have many causes including intestinal parasites, food allergies or intolerances, metabolic disturbances from internal organ malfunctions, viruses, toxins, intestinal cancer or even stress. After a physical exam, your vet may perhaps suggest a change in diet or medications. Make sure they also perform a rectal exam to evaluate her anal glands. A stool sample analysis is a very important part of getting to the bottom of this as intestinal parasites are one of the most common causes.
I can understand your frustration with this problem, but metronidazole is a prescription item and would need to be prescribed by a veterinarian. It is a relatively safe drug, but occasionally we do see cats that have side effects which can be neurologic in nature. It is for that reason that I believe a vet should evaluate her first to make there is not something else that may be effective at treating her signs. If you are up front about your financial concerns with your vet, I am sure they will be able to work with you to come up with a diagnostic and treatment plan within your budget.
I hope this information helps and please let me know if you have any further questions.