I read your question carefully and I understand your concern. Side effects of Paracetamol are thankfully very rare, they include mostly allergic reactions and liver damage, but they are rare thankfully. Taking a drug which has expired carries more of a chance of causing side effects, although still the likelihood of any side effect remains very very low. More likely it may have lost some of its efficacy which could be the reason it hasn t stopped your headache. It is generally safe to use another drug like for example Ibuprofen (caution is needed if you have an ulcer or high blood pressure) to treat your headache. I hope to have been of help.