How long can cat medicine be good for?

Back in February, Tech Cat 2 suffered from an upper respiratory infection.  Along with his antibiotics (which he finished) he was prescribed an appetite stimulant which he did not finish.  He was feeling ill about a month and a half ago, and there were still some appetite stimulants left from the doctor and I gave them to him and he ate and pulled through again without any problem.

Since he's fine and eating well, there are still 2 tablets left that he has not needed to take.  I was just wondering how long this medicine is good for, of course praying that he will never have to take it again.  It has helped him to regain his appetite very well when he needed to.

The official shelf-life for cyproheptadine and mirtazapine is about 2 years, and for valium, I've heard anywhere from 3 to 5 years. But I've used cyproheptadine 4 years later and it was still fine.
 

It's really impossible to say how long your medication will be good for since it depends on when it was made. Although this particular drug may have a shelf life of three years it doesn't mean that it will last for three years for you since it could have been sitting on a shelf at the vet's office for 2 years and 10 months. It may already have expired. The only way to know for sure is to call the vet and ask what the expiration date is for this particular batch. If they keep good records they should be able to tell you.

Normally meds will be okay for a while after it officially expired. There is normally no harm to use expired medication (with some exceptions) but it may be less potent.
 

If you have kept all the original packing you might find an expiry date stamped or printed somewhere on it.  If you've kept the medication in the blister pack then it will probably keep well some time beyond that, but if it was dispensed into a pill bottle then the shelf life is shorter as it's not protected from damp in the same way.