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Most gels take consistent usage in order to see results.  Optimum results have been shown in 20–30 minute sessions for some products.  The minimum time for real results is roughly two weeks, with minimal shade differentiation showing up after a few days.  Maximum results should take at least two weeks, but some programs take as long as four weeks to show the best results.  Longer wear times can potentially create faster results, but also increase the possibility of sensitivity. Results also depend on your enamel structure, which may vary from person to person. Using bleaching gel with a higher concentration has the potential to give you quicker results.   Results are maximized in the first few hours, and you can typically expect the whiteness to lift four or five shades over the course of treatment. If too much of the gel is squeezing out of the top of the tray, the excess is probably causing the sensitivity.  Try to use a smaller amount of gel with each treatment until there's a good balance of coverage and oozing is minimal.  The goal is to have complete frontal tooth coverage without gel waste. Depending on the treatment, it’s possible to use the whitening gel for multiple hours per day or while you sleep.  While the gel loses most effectiveness during the first two hours or so, lessening the amount of time that the gel is touching your teeth could help with reducing or eliminating any pain you're feeling.   If you are using the tray overnight, discontinue using the tray for such long periods. Shorten the length of time your use the tray.  Some treatments call for over an hour of use.  Try lessening the time by about 10 minutes per treatment, or to around 20 minutes. Since the amount of time used is directly related to the effectiveness of the gel, you may want to continue the current length of time the tray is being used.  If you’re experiencing sensitivity, however, you may try breaking the treatments up into smaller periods.  Take a two-hour treatment and divide it into two separate one-hour treatments. A one-hour usage could easily be broken down into two 30-minute periods. The obvious downside to breaking up treatments is the usage of more gel than normal, however, you may also be able to maintain the whitening effect for a longer period of time. If you’re having major issues with sensitivity or pain, you probably need to suspend use of the gel.  In general, you should probably take one or two courses of action.  Skip a few days. Or, skip a few weeks. Most sensitivity diminishes after the first few days, but some instances have been shown to take weeks to subside.  Seek professional advice.  Only a dentist can truly determine the cause of specific tooth sensitivity.  If you’ve discontinued use of the gel, and you’re still experiencing pain, there may be a larger issue requiring an x-ray and/or dental work. While the gel tray is in your mouth, ensure you do not eat, drink, or smoke.  Similarly, refrain from eating citrus fruit just after application.  Improve your results by minimizing consumption of coffee, tobacco, sodas, sports drinks, wine, and tomatoes in your daily diet.  Drink with a straw.  This helps staining fluids to bypass the teeth.

Summary:
Look for whitening progress. Ensure you’re not overloading the tray. Shorten the treatments. Break up the treatments. Suspend treatments and see your doctor. Discontinue or minimize counteractive products.