Different chemical compounds - if they do break down over time - do so by different mechanisms. Back in the late 1970's, the FDA mandated that every drug product bear an expiration date of no longer than five years, even if there were no evidence of breakdown (think of mineral salts, such as potassium chloride). Two of the most common mechanisms of breakdown or decay are moisture and exposure to light (of different wavelengths for different drugs).
Even as a retired clinical pharmacist, I don't feel qualified to offer but one serious caution: when members of the tetracycline family decay, they can produce kidney damage. Hence, these are among the weaker candidates for long-term storage.
Some injectable medications that need to be dissolved before they are injected are stored frozen. Most do not.
If you have a relationship with a working pharmacist, you may be able to ask about the factory expiration dates. Generally, if the container from which the pharmacy dispenses comes with a two-year expiration, that drug is subject to decay. If it comes with a five-year expiration, it will tolerate longer storage. The expiration date on the prescription container is often arbitrarily assigned as one year by the computer that generates the label. Ideally, the pharmacist will double-check the factory expiration date and change the one on the prescription container if the one-year date exceeds that from the manufacturer. The pharmacist should also be able to tell you about those drugs that require refrigeration or freezing before dispensing.
Not to be snide but this does not strike me as the best forum in which to post this question. There is a great deal of drug information available online - some oriented toward the lay person, some oriented toward the various health professionals.