I'm not sure how it works in the EU, but in the US, most states have a "PMP" (prescription monitoring program) that tracks the sale of marijuana in many states (nevermind that its not an actual prescription, but it is a controlled substance) and viewable by your doctor back up to ~12 months or so. Most people don't know this however and think it works like alcohol sales where it's sold after ID verification and then everyone forgets about it. Some states treat marijuana sales like prescription drug dispensing, it has to be reported to a central database including the intimate details of the persons involved. I have no idea if this is the case in Spain, however.