It was not all the time, but started an hour within taking citalopram and peaked at around 3 hours from the dose - I would have to spend the rest of the day eyes closed in bed (I took cit in the morning so whole day pretty much). After sleeping it mostly went away and started again with taking the dose. It was easy for me to put 1+1 together that this was some adverse reaction or side-effect. Also it was not just a headache, it was also in my spine and chest..