Hair Collection Common Hair Testing Myths/Concerns Head hair has a more precise growth pattern, so generally it is preferred over body hair (neck down). A continuous rate of growth of ½ inch per month occurs on head hair. Body hair grows at one-half inch per month, but depending on the person it goes dormant anywhere from 2 weeks out to as long as 4-5 months (even more in some cases). After 10-15 months from the start of growth this hair falls out. For this reason the BODY HAIR results are reported as approximately a “One Year” timeframe, though there is some flex in that timeline. It is important to remember that the BEST application of a hair test is to find if a person has a substance abuse HABIT, it is NOT useful in trying to identify a specific date or narrow timeframe that someone may have used a drug.