The National Headache Foundation compiled a panel of experts in 2000 to review the published evidence regarding the relationship between caffeine and headaches. Here is a summary of what they found.

General

• Most people feel the effects of caffeine within 30 minutes.
• Generally, the effects of caffeine last 3 to 5 hours.
• The average American consumes about 200-300 mg of caffeine daily, or the equivalent of about 2-3 cups of coffee.
• People who get headaches should clearly understand how caffeine affects their headaches.

Caffeine in Headache Medications

• Adding 130 mg of caffeine to a regular, two-tablet dose of common ingredients found pain relievers (aspirin and acetaminophen) makes them relieve headache pain about 40% better than they do without caffeine. Caffeine also helps your body absorb these medications, allowing you to get back to your daily life faster.
• Because analgesics work better when they have caffeine added, you may be able to take less medicine when you have a headache. And because even non-prescription medications are real medicine with the potential for side effects, taking less reduces the risks associated with inappropriate use.

Caffeine and “Rebound” Headache

• “Rebound” headache is a serious problem that develops from taking too many headache medications too often.
• Rebound headache is constant -- it won't go away until you completely stop taking the drugs that are causing the problem.
• Any headache medicine can cause rebound headache -- taking caffeine-containing headache medications doesn't appear to increase the risk.
• Rebound headache is rare; in fact, less than 2% of the general population is reported to suffer from rebound headache due to over use of pain relievers

Addiction or Dependence

• According to both the World Health Organization and the American Psychiatric Association, caffeine is considered safe, and in no way resembles other addictive or habit-forming drugs that lead to severe physical and social consequences.
• Significant caffeine abuse has not been reported by any culture in the world.
• When used according to label directions, headache medicines with caffeine pose no risk of addiction or dependence

Caffeine Withdrawal Headache

• Technically, a person needs to use caffeine every day and reach a monthly total of at least 15 grams before they will experience caffeine withdrawal headache.
• Withdrawal symptoms occur when people consume 500 mg (about 5 cups of coffee) or more daily, but they have been reported after long-term, daily intake of 100 mg or less per day

Source: “Debunking Myths: Fast Facts – Caffeine and Headaches”, National Headache Foundation