Not sure why this is downvoted, since this has been a working legal loophole for decades. For example, 1cP-LSD is completely unscheduled in the US right now, despite breaking down into pure LSD once consumed (at only slightly reduced potency). It's already banned in several other countries though.

Though, I am sure prescription drugs will always be legal with a prescription, assuming the prescription itself is legal. Even if this were counted as the Schedule I drug LSD that can't be prescribed, it'll probably get moved to Schedule II or III before any actual prescriptions are given out. So anyone who ever gets such a prescription in-hand will always be safe.