The best is to avoid anything but water, as your main goal is to eliminate every single possible allergen. Even water should be as clean as possible, like at least filtered tap water, or natural spring water you get from some good quality source.
The fact is that you do not really know what you react to, you are blind, so the best is to assume you may react to just about anything from your day to day life, including deodorants, perfumes, soaps, shampoos, shaving foams, gliding soaps added to shaving blades, just about anything.
If you do not eat, you do not need to brush your teeth with toothpaste, it is enough to use clean water.
For majority of people who embrace water fasting, symptoms of allergy start fading within the first 7 days.
The more corticosteroids, Antibiotics , artificial hormones, birth control hormones and other medications person have used before starting the fast, the longer it can take to become symptom free as a person may be dealing with steroid addiction.
Having any artificial implants or anything artificial inside body during a Water Fast may prolong time it takes to become symptoms free.
Some people have even fasted for 60 days (morbidly obese people) but an average Joe needs usually just 7 - 14 -
21 days to become symptoms free.