Today, you see some pain relievers with caffeine, which is supposed to “help” get rid of headaches. They say it opens your blood vessels or something. However, I have read that caffeine restricts the blood vessels in the brain, and once the caffeine wears off, the increased blood flow to the brain gives you a headache. This can be a marketing scam to make you use the caffeine acetaminophen pain reliever more once the headache comes back. Which is it? Help or hurt?
Purely anecdotal, also, IANADoctor or Pharmacist:
When I had a prescription medication for my migraines, I had Cafergot – essentially caffeine and ergotamine. As I mentioned in this thread, I now just take Excedrin, which is caffeine, aspirin and aceteminophen (better known as Tylenol). The caffeine helps me greatly and caffeinated beverages have been a part of my daily life for as long as I can remember (I’ve been a comitted tea and coffee drinker since, well since forever). The vascular narrowing you mentioned is why it specifically helps migraine. Migraines are enlarged and inflamed blood vessels in the brain.
On the other hand, you hear horror stories of caffeine-dependence and the migraines that it can cause. So it seems, purely in my own experience, that caffeine is helpful for migraine – to a point. I imagine individual experiences also vary. For regular headaches, I imagine that standard OTC medications would be most efficacious.
Drinking a Soda helps me when I have a headache, but i’m willing to bet it’s because i’m addicted to caffeine.
Yes, caffeine helps. As does caffeine + sugar (soda).
Unless, you are drinking it everyday then stop…that can cause a headache. Caffeine does constrict the blood vessels in the brain.
My vote: not a scam.
Caffiene can help certain headaches. It certainly reduces a caffiene withdrawal headache! 
But it can also help lessen some tension headaches, and if used early in a migraine attack can help to abort it. Best used with a painkiller like acetaminophen or NSAIDS.
QtM, MD
I get a headache maybe once a month. Coffee will make it better or make it go away 90 percent of the time.
My son cannot drink anything with caffeine or he will get a headache, often a migraine.
Final note:
Read the directions regarding ingesting caffiene from pain meds in addition to intake from colas, coffee, etc.
If the headache is due to dehydration I’d imagine that a diuretic like caffeine would make it worse.