Hello,
Welcome to Healthcare Magic and thank you for writing to us.

There are usually no side effects to taking painkillers past their expiry date, the only thing that does not happen is to cure the pain, in your case lower the headache.
However, if you had taken several over the day without looking at the expiry date, side effects of nausea, vomiting are likely. Some people have complained of feeling 'spaced out' but more so after they have taken a lot and then figured out the tablets had expired!

So unless you are having any symptoms as of now, I would not be worried. Take a different analgesic such as ibuprofen or voveran but after food. Do not take aspirin for headaches. The safest is paracetamol (or panadol) that you had tried.
Regularly check the expiry dates of medicines at home, put them all in a bag, and throw them away instead of pushing it further back in the drawer.
Most important, check what medication you are about to take, and then the expiry date.

I hope that was useful.
Best Wishes.

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