It's unlikely that you'll experience any discomfort when having your teeth professionally whitened. Some people find their teeth and gums may become more sensitive during treatment; however, you can use toothpaste designed for sensitive teeth to reduce any discomfort and your dentist can advise on how to manage any sensitivity.
In rare cases, teeth may be very sensitive during treatment, but they will usually recover when the whitening treatment is stopped. At mydentist, we also offer a product called Philips Zoom!, which contains an ingredient called amorphous calcium phosphate (ACP), designed to help reduce sensitivity significantly, and provide extra protection for your enamel, improving the final shine.