When I took my drug test for my current job I was about a week out from using codeine cough syrup for a nasty respiratory tract infection. I told the hiring manager, and they said that there would be no problem at all as long as I had a prescription. If your levels are high enough that they suspect that you will be impaired on the job, that may be an issue, but if you have a valid prescription you should be fine, and you should probably talk to the hiring manager before you come complain online.