Have bad headache on the left side of the head?

Migraine or more: If headaches are always one-sided be suspicious of causes other than migraine, which is frequently one-sided. If even 1% of headaches are right-sided, you have less to worry about. But 100% of headaches being one-sided (either right or left) suggests a structural problem--which may be quite serious (avm, tumor, aneurysm, vasculitis, cyst, skull or sinus lesion, etc).