Migraine 

A migraine causes the sufferer severe headaches, often accompanied by disturbance of vision, nausea and vomiting. Attacks are triggered by a variety of things, from flashing light to food allergies and even certain smells. 20% of all people are estimated to suffer migraine, and woman are generally more susceptible than men.

Cannabis was highly regarded as a treatment for migraine in the 19th century. Dr J.B. Mattison wrote in 1891 that the treatment of migraine was the most important application of cannabis. 

In addition to the pain-killing and anti-nausea properties of cannabis that help when an attack is underway, there is also evidence that it can deter an attack at the earliest warning signs (usually flickering vision). The neurotransmitter serotonin may be related to migranes. Some reports suggest theat THC only inhibits the release of serotonin from the blood during the attack, being otherwise non-reactive with this neurotransmitter."