After having some personal experience with doxycycline that had gone past its expiration date, I think this topic needs a little more research and a lot more specificity. Be careful. Putting expired drugs into your body may be hazardous to your health.
I've always understood that tetracycline and some of it's derivitives can be liver harmful once they deterioate. I suppose the timeframe of deterioration after “expiration” is relative to storage conditions. Having never seen any specific guidelines on how that toxicity timeline works, I've ALWAYS abided by the “expiration” date on the cyclinic drugs.