THC dilates peripheral blood vessels ( that's why you get red eyes) and some varieties can dilate blood vessels in the meninges ( the covering of your brain) which can cause a headache. I suspect there may be a predisposition in some people towards this. It's harmless but very uncomfortable.

If you tolerate them, you can take a NSAID like aspirin or ibuprofen (Advil, Motrin) before you indulge and see if this prevents it. Maybe check with your doctor as to the best preventative.

I've been told that Indicas are less likely to cause headaches. I only use prescription THC ( which can definitely give me headaches) so I have no experience in this. Anybody?