I suspect that many of us have shut away in the recesses of our chicken medicines a few out of date horrors but I'm particularly addressing antibiotics. I bet there are loads of us with a bottle tucked away with the last few drops of those precious abs in them. They are so difficult and expensive to obtain in the first place that I can hardly condemn the practise (I keep mine in the fridge!).
However I would like to sound a word of caution. A/bs indiscriminately used in inappropriate situations can be far more dangerous than benefitial. First of all you should never use a/bs for a purpose other than for what they were prescribed on the off chance they may help. Used for the wrong purpose they can reduce the bodies natural immunity without having any effect on the problem they are being used for. Thus they can easily make matters worse. If it is a case of a reoccurrance of the condition they were prescribed for I suppose there's some excuse but make sure that they are within their effective date and they have been stored properly in the meantime.
Secondly never start a course of treatment unless there is sufficient to complete the course and that is usually 5 to 10 days, don't start it. If you give a partial course you may well be helping to breed a resistant strain of the infectious organism to the detriment of all animals. Exposing the organisms to a none lethal treatment is asking for trouble.
Unless you have some knowledge and experience with a/bs I'd strongly recommend you dispose of them safely once used for the prescribed purpose. If you have an understanding Vet who is willing to prescribe repeat doses at a reasonable cost there should be little need to run the risk of leaving the remains of those tempting wonder cures in the medicine chest.
HF