How long is the shelf life?

Are you asking about a specific medicine?

Every medicine has a different shelf life which corresponds to the expiry date the manufacturer puts on the bottle, with the batch number.

If the expiry date is missing and you have a batch number, you can find out the expiry date from the manufacturer.

Nitro is very touchy... hot weather, opening of bottle, many things can to effect the quality. You should always carry 2 bottles... one that your using and one new one... if you take a nitro or two and do not get an effect, immediately open the new bottle and take it. Old nitro or ineffective nitro could cause you to go on to have a heart attack. Been there so I know...