Some medicinal compounds are hygroscopic and may go bad over time due to the permeability of moisture through the packaging. (Hence the use of metallized foils,

Some medicinal compounds are hygroscopic and may go bad over time due to the permeability of moisture through the packaging. (Hence the use of metallized foils, which have nearly zero moisture permeability). Same for oxygen/oxidisation. But yeah, depends on how far past expiry you go.