Regarding use of the Fluoride Trays, I have noticed there seems to be differing instructions.
I am not sure, but perhaps this is due to the fluoride being prescribed.
Below is copy of the instructions I was given (OCR scanned).
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Sharp Grossmont Hospital: David and Donna Long Cancer Center
Harold J. Gulansen D.D.S. / Peter F. Johnson, D.MD.
Diplomate American Board of Prosthodontics
Prosthodontics, Maxillofacial Prosthetics, Implant Reconstructions

INSTRUCTIONS FOR THE USE OF FLUORIDE TRAYS
1. Brush and floss your teeth
2. Place enough fluoride gel to just cover the bottom of the trays
3. Place the fluoride trays over your teeth as directed
4. LEAVE THE TRAYS ON YOUR TEETH FOR 5 MINUTES
5. Remove the trays and spit out excess fluoride gel
6. DO NOT RINSE YOUR MOUTH, EAT OR DRINK ANYTHING FOR 30 MINUTES
7. Clean the trays with a toothbrush or cotton swab
8. Place the trays in water for storage

USE THE FLUORIDE ONCE EACH DAY, EVERY DAY, EVEN AFTER YOUR RADIATION IS COMPLETED.
As a result of radiation, most patients experience dryness in the mouth. A certain amount of dryness will probably persist forever after radiation therapy to the head and neck. The most common problem associated with a dry mouth is rampant dental decay and periodontal (gum) disease. DAILY application of fluoride has been shown to be effective in preventing these problems. If a patient. is not compliant, dental disease, oral infections and infections of the jaw bone are common problems.

If you have any questions, please call our office for assistance.

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FYI:
The fluoride I am using is OMNI GEL 0.4% Stannous Fluoride / Made by: 3M ESPE
I have been using Grape flavor, one of several flavors available and I pick it up at the CC Prosthodontics office for $12 per 4.3oz bottle. A bottle lasts about 3 months.
I was told any Fluoride Gel of about 0.4% to 0.6% is fine to use and generally available without a prescription from most pharmacies.