What is the duration of the telogen phase on the back (longest)?

• A. 3 months
• B. 6 months
• C. 9 months (longest)
• D. 12 months

Answer: (C)

Hair grows in cycles: anagen (growth), catagen (transition), and telogen (rest). The length of the resting phase varies by body area because follicles cycle at different rates. The back has the slowest follicular cycling, so its resting phase is the longest. On the back, the telogen phase can last about nine months, longer than on other areas. This matters in laser hair removal because lasers are most effective when hair is in anagen, so the longer telogen means hairs pause longer before re-entering growth, requiring multiple treatments spaced weeks to months apart to catch hairs as they cycle back into anagen. Three or six months are shorter rests, while twelve months is longer than typical for the back; nine months best represents the longest duration.