It was my understanding that it is included because many people taking headache pills don't actually realize that they are caffeine addicts and don't recognize the withdrawal symptoms for what they are. This makes the pills more effective in making that class of headaches go away by eliminating the withdrawal symptoms, sort of how heroin is an effective cure for heroin withdrawal.
> It was my understanding that it is included because many people taking headache pills don't actually realize that they are caffeine addicts and don't recognize the withdrawal symptoms for what they are.
Many things about the OTC drug market in America are rather disturbing. 26k people per year are hospitalized from acetaminophen poisoning, and about two die every day. It's the scumminess of the pharma industry, with less adult supervision.
As with many things, all the suggested reasons apply, in different proportions for different people. The caffeine may help with vasodilation-related symptoms. It also may help with any other caffeine-withdrawal symptoms, if present. And it may create a slight new dependence – as with the other painkiller ingredients, which can contribute to 'rebound headaches' with persistent use. Adding an addictive yet legal substance to your product is rarely bad for repeat business.
No, it's because it's a vasoconstrictor. As a person who is both a caffeine addict and a past sufferer of angina pectoralis, I can testify that the headaches caused by caffeine withdrawal and the headaches caused by nitroclycerine (a vasodilator) are one and the same.
Vasodilation is not necessarily chemically induced, but it is a common-enough proximate cause of headaches (and some other forms of pain) that using a mild vasoconstrictor in addition to an analgesic is more likely to make a general-purpose pain reliever work effectively.
If I remember correctly, caffeine's purpose in migraine medication is to accelerate the delivery of the payload.