Absolute genius Will, that's exactly how you should do it.
Get an offer with a coupon code and print it on the leaflet. I'm betting this will massively increase your leaflet retention, as people always feel uneasy about throwing away a freebie.
Otherwise, you're just reminding them that they need to get their teeth whitened. They'll just throw away the leaflet and call their dentist/search google when they get home.
Give them a strong reason to put that leaflet in their pocket.
I'm guessing this will be a fail without the coupon approach, but if it is, don't give up and until you've tried the coupon angle.