Harmful? Probably not. But ineffective? Oh yeah. Long-used rule of thumb for aspirin: Remove the cap and sniff deeply of the bottle. Is there a strong (not "faint") vinegar smell? If so, it's useless - throw them out. If the vinegar smell is very faint, it's still effective.
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Some medications literally undergo chemical reactions over time, and that's why they expire. In general, if there's an expiration date then there is a REASON for the expiration date, and you are better off not taking expired medication.
Yes, their effective ingredients may no longer be effective and therefore they will not cure whatever you are taking them for AND more worryingly, their ingredient may have degraded/changed into something that is now harmful to you (for instance, out of date aspirin does this).
If you are ill, go and see a doctor and get new fresh medication prescribed for you.
Not physically, but they may not work on the problem that you are taking them for because they are expired.