Is it harful to take Tylenol after the expiration date?

Some drugs lose their potency over time. Others can decompose into potentially harmful compounds, though this is rare. It's usually a matter of effectiveness.

The reason that most drugs remain effective long after the expiration date is that the FDA requires the drugs to be tested for effectiveness over time. Most drug manufacturers don't test beyond a year or two past the date of manufacture. It's like a warranty period. After the expiration date, the drug is "out of warranty". But it might still work just fine.

Drugs like aspirin and Tylenol have been around for many years, so why the short expiration? We know they have a long shelf life. It's because every time the manufacturer changes its packaging or formulation (pill shape, pill type (tablet vs capsule), and a variety of other things), the testing process has to start over from time zero.

So the expiration date really only states how long a particular formulation has been tested, and how long the manufacturer is willing (or the government requires) to guarantee its effectiveness.