Friend had a sudden very severe headache in the back of his head--er tests indicate not tumor or aneurysm . any other ideas re. possible cause?

Occipital headaches: Headaches localized to the back of the head or the "occipital region" can be varied and numerous. Possibilities include occipital neuralgia/neuritis, migraine headache, basilar migraine, cervical radiculopathy, and subarachnoid hemorrhages. And there are more. The best advice i can give is that your friend needs to see a physician interested in headache work ups and that a diary should be kept.

Depends: First off, er typically will do head ct with or without lumbar puncture. We call sudden onset of a severe headache a 'thunderclap headache'. Er docs want to rule out aneurismal bleeding because this can be immediately life threatening;however, there are other causes. I'd recommend that your friend see a neurologist for further work up.