As quoted from http://www.opalescence.com/takehome.php
"Opalescence Take Home Whitening Gels allowyou to whiten your teeth at your convenience—in asafe, effective, and customized way.Safe
Opalescence Take Home Whitening Gels are only available from your dentist, who will supervise your treatment from initial consultation through completion. The sticky, viscous gel won’t migrate to gums, and most formulations include Potassium Fluoride (PF), which helps to improve oral health by strengthening enamel, decreasing sensitivity, and offering increased cavity prevention.
Effective
Powerful, prescription-strength Opalescence Take Home Whitening Gels are available with 10%, 10%PF, 15%PF, 20%PF, and 35%PF carbamide peroxide. Opalescence Take Home Whitening Gels can provide results after just one night, though expect to see results in about a week.
Customized
Opalescence Take Home Whitening Gels are delivered via customized bleaching tray. Your dentist will create this tray from an alginate impression of your mouth; this customization is part of why Opalescence Take Home Whitening Gels are so effective. You can further customize your Opalescence Take Home Whitening Gel experience by choosing from the five formulations and three flavors (mint, melon and regular)."