I don't think you can patent and steal a monopoly on a drug synthesized in 1912.
The first is the very high cost ($ billions) of getting through FDA approval for a drug. Once a company has done this approval, then other companies must show drug equivalence in order to produce generics that can stand in. They have to wait for the patents to expire, or figure out how to get around existing patents, but that's still not enough. They must show that their drug is equivalent to the existing drug. But this is not easy task, because the incumbent will do everything they can to prevent anyone else from getting the drug for research purposes. (e.g. the incumbent will claim that a particular company trying to get the drug does not fit their safety protocols, and thus refuse to provide the drug. Because the company cannot get access to the drug, they can't prove equivalence.)
Another method is that a prescription for one drug does not always guarantee that the prescription can be used for a generic drug of the same sort [0]. The incumbent company will spend lots on marketing by buying lunch for doctors, giving them notepads and pens, etc. Then, the doctor will write a prescription for that drug as write "Dispense as Written." This prevents the pharmacist from providing a generic alternative, even if they're biologically equivalent!
A final method is to develop a drug with a particular injection mechanism. For instance, if a drug company puts MDMA in a patch, then they can get a patent for the combination MDMA + patch, get approval, and then get doctors to prescribe it. (Maybe it provides a small dose over a long period?) This can occur even if MDMA was first synthesized a century ago. The prescription for the patch cannot be used for a pill form, or vice versa. This guarantees the drug company revenue as long as they can convince doctors to prescribe it.
So, to make a long story short, there are a lot of ways that drug companies guarantee their $$$ and prevent competitors from eating their lunch.
[0] https://www.washingtonpost.com/national/some-doctors-insist-...
First off, getting a new drug approved is not "preventing generics". Generics need a new drug approve before they can get their generic approved (since they based their application off the original drug's data). This is a feature, not a flaw of the system. Without some guarantee of market exclusivity, nobody will spend on R&D to get approval.
Second, yes, a doctor can write "DAW - dispense as written", however most insurance companies won't cover the originator drug if a generic is available. The patient is then on the hook for the entire cost. That's a strong incentive to go generic.
Finally, unless the new administration method (patch, etc) has some benefit over the generic, doctors won't prescribe it and insurance company's won't cover it.
I've heard rumors of the insurance companies backing off of this stance in the past few years. Essentially, since the ACA puts a hard cap on profit margins, insurance companies are looking for ways to spend more so they can have the same margins on a bigger pie and still increase real profit year after year. Policies like relaxing generic vs proprietary let's them achieve that in a controlled manner.
Just look at the telecommunications industry.
The drug development ecosystem in America makes me sad.
They use specific salts and specific isomer ratios in order to make it patentable.
Small modifications can have that effect. It's why we see so many sulfonamide drugs... small modifications that increase the efficacy of the drug.