99% of drugs that are used past their sell by date, which is what most manufacturers try and convince you is an expiration date. Keep in mind this date is also shorted typically by 2-5 years depending on the drugs. Viagra and the other ED's have a 2 year shelf life, before losing their effectiveness, but do not increase toxicty.
So, the drugs life for the on the shelf is a sell-by date, or more typically begins to lose its potency - the longer past that date, the slowly its loses its effectiveness.
And while some drugs can become more toxic Viagra and their competitors are not in this class of drugs. They only lose some effectiveness, and after a number of years will finally just be rendered inert and have no effect on you i.e. lacking the desired results one is taking the rx for.