Before it was Zyban, this smoking cessation drug was the anti-depressant drug, Wellbutrin.
In past posts where I have shared my drug company ire, I have explained the legality of off label drug use and the unlawful practice of marketing a drug for an unintended/untested use.
The present situation could have gone something like this: After Welbutrin was approved in 1985, doctors prescribed it to their depressed patients some of whom were smokers. A fair amount of these depressed smokers stopped smoking. The doctors might then have decided that other smoking patients, even ones that were not depressed, should try the pills (never mind the egregious side effects of psychotropic meds). Or it might have gone like this: A doctor goes to a fancy schmancy dinner hosted by GSK (maker of wellbutrin) and is told, "Hey - this drug can help people quit smoking, treat sex dysfunction, and more." Then the doctor starts to prescribe it for those off label reasons.
That is basically what GSK has been found guilty of and fined 3 billion dollars over - along with other drug marketing and suggesting adult approved drugs for use in children.
Zyban did come on the market in 1997 and is a first line on label cessation drug. To clarify - a doctor can use an FDA approved medicine to treat anything he deems - but a drug company cannot in any way encourage him to do so.