Can I substitute 1000mg paracetamol with 400mg ibuprofen for a headache?!

You can definitely make the substitution. Paracetamol and Ibuprofen are both pain relievers with Ibuprofen also being an anti-inflammatory. The two drugs work on different pain receptors, so you may find that one is more effective than the other or you may find them equally effective. You can take up to 800mg of Ibuprofen at once, but I would not recommend doing so the first time you take it. Taken in higher doses Ibuprofen can be hard on the stomach and should not be taken more than every six to eight hours unless directed otherwise by a physician.

Registered Nurse