Mostly expired medications can lose potency and efficacy, rather than cause toxicity. However, some medications can have toxic effects past the expiry date. I remember once Roche did some tests on their pharmaceuticals.
But liquid antibiotics (especially the ones reconstituted for oral use) can cause bacterial contamination past the expiry date (non-sterile water used to mix these), certain eye drops can be affected this way. Also Nitroglycerin is easily degraded due to environmental conditions.
Once a drug loses it's efficacy the treatment regime is not going to be very effective. So it's not only a matter of toxicity alone.
A long time ago when Tetracycline was widely used, it was reported that the expired Tetracyclines caused a reversible form of Kidney damage (Fanconi Syndrome) due to Tetracycline degradation. However, the later drugs which are derivatives of Tetracycline family did not exhibit this issue.
General FDA directive for patients is that -
"If your medicine has expired, do not use it."
The scoundrels who do this should be severely punished. But I am aware that repackaging happens in SL.
