I just scrounge small plastic bottles (usually from a super small sized bottle of aspirin, Tylenol, etc.) And I refill those from my larger at-home stock. They're fine for a year (probably much longer).

After I've decided they're starting to get old, I take out the meds and put them back at home labeled "take these first". Then I refill the little bottles from my at-home stock and put them back in my mobile kit.

Half the time I forget to do the yearly refilling. But you know what? Five year old Tylenol works just fine for me. Same for Advil. I rarely take aspirin, and at five years old it's definitely smelling vinegary, but it will still work and isn't going to hurt you. Just consider your five year old 325mg aspirin tablet to be effectively a 315mg tablet. Good enough.

This is not good practice for prescription meds that are sensitive to dosage, or that degenerate quickly. But for over the counter meds? Home rotation like I do above is just fine IMHO.

If you're worried about it, you can probably buy a whole bottle of 100 aspirin to drop into your kit, and that would cost about the same as a miniature foil-pack of four aspirin. Those little packets are not exactly cost efficient...