The concept of "controlled substances" has a special meaning, and stems from the Comprehensive Drug Abuse Prevention and Control Act of 1970, resulting in Chapter 13 of Title 21. Ordinary presecription drugs are the result of an older act, the Federal Food, Drug, and Cosmetic Act, which is also implemented in Title 21. The basic distinction is the difference between "potential for abuse" vs other stuff, and potential for abuse was the historically first desideratum in restricting pharmaceuticals in the old days. There are 5 "schedules" (lists) regarding controlled substances, ranging from the absolutely forbidden Schedule 1 (weed, 1-pentyl-3-[1-(4-methoxynaphthoyl)]indole which probably has a shorter street name) down to Schedule V small doses of opium.
Pharmaceuticals fall into three categories: over-the-counter, prescription, and controlled. Things like ibuprofen over 200 mg is not a "controlled substance" though does require a prescription, as do anti-cholesterol drugs (since they are thought to pose "some risk"). There are also completely unregulated substances, such as green tea, olive oil, vitamin c, which so far are not subject to federal regulation (since they have not been deemed to pose a risk). OTC substances can be bought without special hoops, though they cannot be marketed without passing through hoops. Prescription and controlled require a prescription from a DEA-authorized physician. Ephedrine and pseudephedrine are a recent zombie class of pharmaceutical which do not require prescription, but are subject to purchase quotas.
Controlled substances are subject to much higher levels of scrutiny, to ascertain that the prescription is legitimate and the recipient is the intended recipient; there are also stricter laws about possible justifications for prescribing regular prescription drugs vs. those on one of the 5 schedules. (And the rules are different for the 5 schedules).
It is generally unknown whether doctors can prescribe non-prescription pharmaceuticals, in order to shift the cost to insurance. There is no specific statute prohibiting that, but doctors are also subject to contractual restrictions between themselves and the insurance networks they deal with, so they may have agreed to not engage in such conduct. As far as I can tell, it is not actually illegal for a doctor to write a prescription for OTC Tylenol.