It used to be fairly normal, the pharmasists knew the various doctors in the area, and they also know what is a reasonable prescription.
That just seems like a system that is broken by design. If the pharmacists know what a reasonable prescription is, then why bother with the prescription pad at all? Just have the patient ask the pharmacist for whatever it was the doctor recommended.
I suppose what probably happened was that initially the prescription pad was just any random scrap of paper and the doctor wrote down the prescription so that the patient didn't have to remember the exact details. But, then drugs started getting more powerful, and people started abusing them, so what used to simply be a note to help the patient remember became a secured way to authorize the pharmacy to dispense something.
If the system had needed security right from the start, it probably would have been a system where the doctor sent a prescription directly to the pharmacy via a courier, a phone call, a telegram, or something.