By definition, a prescription medication has been approved by the FDA. This means that an independent, third-party has evaluated the drug. There are three dimensions that the FDA validates.
Efficacy: prescription drugs work. They've been scientifically demonstrated to have the impact that they claim by doctors, scientists, and regulatory agencies.
Safety: they are safe in humans. If they have side effects, they are documented, known and manageable in the context of appropriate clinical use. There are no "surprises."
Purity: pharmaceutical compounds, and the pharmacies is use them create them, are heavily regulated: the ingredients are pure. You get what is prescribed, and only what is prescribed.
Do Prescriptions Really Matter? Dr. Falola Responds.
Why Prescriptions Are King