Mouldy medicines

When drugs pass their expiry date, when this is not a function of alienation of value, they presumably change their chemical structure to some extent. To take a non-medical example, sugars become ethanol and ethanol becomes ethanoic acid, each of which is pharmacologically different. Similarly, inactivated pharmaceuticals in sewage can be reactivated by microorganisms or, presumably, otherwise altered. Then there are processes like oxidation, attack by fungi and so on, and when attacked by the right fungi, antibiotics would be produced, as with a bread pill attacked by Penicillium for example.

Two things about this:

i) In certain circumstances the active ingredient will fairly or very predictably become a different active ingredient, or compounds not regarded as active will become so. At such a point, expired medicines would not be so much useless as different, perhaps harmfully so but even then useful as pesticides or something unrelated to biological activities. Therefore, once subject to testing, these could be reused rather than disposed.

ii) Taking this a bit further, drugs could be designed specifically to alter in predictable and useful ways, or otherwise encouraged to change in those ways, for instance by applying certain biological processes to them.

So, basically two drugs (or other useful compounds) for the price of one.