[randomdata]

On the other hand, marijuana has been used to treat those symptoms. It may be your friend had some other problem that was being masked by the prolonged marijuana use.

Or one of the other many reasons why there can be correlation without causation.

[lightedman]

The headaches are caused by the brain not producing its own canabinoids due to the influx of outside cannabinoid sources.

Source - lifelong medical cannabis patient.