Power
Pharma reps are in the driver's seat, really. They're the ones who educate the doctors. By doing this, pharma reps are able to market their drugs effectively. So while it's not quite as direct an approach as telling someone to open up and say, "Aaah," pharma reps do have a behind-the-scenes clout in combating human illness and promoting human health.
Let's face it: Doctors are overworked and have very little time to stay on current prescription drugs. Just turn on a TV for a little while, and you'll hear all about the brand-new drugs that are constantly entering the market, and each one of those represents another potential homework assignment for the good doctor.
There are lots of nitty-gritty details that they need to know about these wonder pills, like their mechanism of action (how they work), their side effects, and clinical trial statistics. And in you step, an encyclopedia of knowledge, and the docs (and ultimately their patients) will thank you for doing their homework for them.
That said: use your powers for good. If your sales are lagging, you might be tempted to put the pressure on doctors to buy, or to bribe them with things like free lunches (source). Just remember that you're selling drugs, not balloon animals. Unlike balloon animals, drugs aren't harmless, and it's up to you to be responsible with the power you've been given.