I read about this on EDS forum. Sounded weird but there was a huge thread on it, including posts by dental hygienists, who approved. Apparently drops like Eurax contain the same active ingredient as the really expensive gels you can buy especially for whitening teeth.
So I have been giving it a go. The drops aren't gross at all, just minty. I smear them over the front 'smile' teeth. Leave it one 10 mins whilst get on with other stuff then brush it off. I must say it has made a huge difference to my old beige teeth after only 3 days! And it's easy and dirt cheap, no faffing with trays and the like!