The answer is Omeprazole could be causing it, or it could be something else! Omeprazole causes me bad headaches, like a sort of band squeezing around the head. I never used to get headaches. The headaches are worse when I alter my dose (up or down) ie a chemical change and are not too bad when I stay level. You should be used to the chemical after 4 weeks, but drugs affect people in different ways. Headaches are common with ppis.