My teeth were not awful before I started whitening them but they were fairly yellow. Now I don't know all the technical details of how a dentist goes about whitening their patients' teeth so I will just talk you through the experience through my eyes. Now there are different ways of whitening teeth but mine was done through trays and bleach. So what happened was a mold was made of my teeth and then through that my trays were made to fit my teeth perfectly. It does happen that the mold won't be totally accurate so it has to be redone several times which takes a while. The mold is made by the dentist putting what feels and looks like blue play-doh in your mouth - which after they have removed leaves you with a really attractive blue beard (until you wash it off). Anyway, so the trays are made and then you use bleach that you put into the trays and you wear the trays at night. There are different strengths for the whitening bleach and mine was strength 10 which I think is the strongest you can get in the UK. Now they aren't super comfortable to wear but you get used to it, the real pain is the next day. You can space it out but I think I whitened my teeth for 14 nights in a row. So the day after you have worn the trays, if you're anything like me then your teeth will be extremely sensitive and the slightest touch of a fabric or squeaky sound will make your teeth feel so sensitive and you'll clench your teeth together... I can't explain how horrible sensitive teeth are unless you've experienced it. From my experience, toothpastes like Sensodyne did not help with the sensitivity in the slightest. However, after a while I noticed that my teeth kind of became immune to the pain and the sensitivity didn't come anymore. Nine months or so on I still occasionally top up the whitening by leaving the trays in for a night or so. When it comes to prices I think it costs around £200-£400 depending on where you go. I would recommend you see a dentist to whiten your teeth rather than get it done at a spa where the people won't necessarily be trained in dealing with certain medical situations or complications that might arise such as an allergic reaction.
All in all, although my teeth went through a lot of discomfort at the beginning, I would say that whitening my teeth was one of the best things I have done. I know that sounds incredibly superficial but I think you feel a lot more confident when you feel like you have a nice smile and you feel good about yourself. I used to never smile with my teeth and now I'm all over that. It also means you can wear orange lipstick more easily!