When is a persistent headache a matter of concern that should warrant seeing a doctor?
If it has been happening over a long time, you need to see a doctor. Other cases are if it is gradually increasing in intensity and getting worse despite taking painkillers. An example is if you find yourself moving from a mild painkiller like paracetamol to stronger ones that have codeine, an opioid.
Additional symptoms like blurring of vision, dizziness and weakness on one side of the body should also be of concern.