OTC stuff will never be enough to actually whiten your teeth. If you have severely yellow teeth, your issues might be more than just stains. You should see a dentist about the overall health of your teeth. If they are healthy enough, get a professional teeth whitening system that includes molded trays to fit your mouth properly. You can probably expect to pay $250-$500 depending on where you live, what dentist you see, etc. Insurance usually doesn't cover this.