What other conditions have a side effect of headaches?

That's a hard question. There are over 300 different types of causes of headache, and the majority are what we call secondary headaches, meaning some other illness or disorder causes the headache. And so examples of secondary headaches can be headaches due to a fever or some other infectious cause or illness, could be brain tumors, could be disorders of the blood vessels, arteries, and veins of the head. Metabolic disorders, thyroid disorders are linked. Anemia can cause headaches. Kidney disease is associated with headaches. So there are a large variety of secondary disorders that are associated with a headache.