I suppose "Big Oil" puts a gun to your head and makes you fill up at the gas station too?
Poor comparison. For the metaphor to make sense, you'd have to be a gas reseller. Let's say the large oil companies came out with a new product. In order to force the gas stations to sell it, all they'd need do is raise the prices of the standard products slightly, unless the gas station agrees to sell some amount of the new product. If the station hits that nut, the standard products get 'discounted' down to the normal, regular prices. Otherwise, if the station doesn't agree to sell the new product or doesn't sell enough, they're paying premium prices on the standard products and forced to raise their retail prices slightly, causing customers to get gas elsewhere to save a dime.
Patients generally aren't doctor-shopping (unless they're already addicts). Pharmaceutical companies offer inventives to doctors/hospitals using similar tactics. A good case-study of what the GP seems to be talking about is OxyContin. This is a relatively new drug formulation that could have been a miracle drug in specific instances of chronic pain or terminal patients, but Big Pharm pushed this pain-killer in a huge way for years under the FDA's radar, giving doctors incentives to overprescribe, and within a few short years on the market we have pregnant women robbing pharmacies to get at it. Big Pharm and the physicians they manipulated effectively created a new narcotic drug epidemic and were successful in massively boosting sales and profits on a drug formulation that initially had a limited application. Subsequently, the drug has been again reformulated, hopefully in a way that is a disincentive for abuse, but new stories are now coming out that its been replaced on the street with a different new drug, a time-release pill formulation of synthetic morphine... once again, a formulation that is completely unnecessary with questionable benefits to legitimate patients and is already proving to have an enourmous abuse vector. But Big Pharm needed a new synthetic morphine because the old synthetic morphine's patent had expired, which reminds us of why they needed synthetic morphine in the first place, because formulations of regular old morphine (which still works just fine and is regarded as a gold standard of severe pain relief) predate the drug patent system by a century.
Once you're hooked to a strong narcotic, no one needs to put a gun to your head for the body to want the drug. It changes the chemistry of your brain forever and the craving for it never goes away.