I am asked this question all the time!!
It’s tricky to answer because it’s not at all obvious.
The short answer is: the interval should track telogen duration, not anagen duration. Anagen tells you what you can kill today; telogen tells you how fast the next treatable cohort arrives. See what I mean?!?!
The Merhag & Richards’ data gave us the following numbers forthe duration of the telogen phase and the percentage of follicles in anagen:
| Site | Telogen duration (approx) | % in anagen |
| Lip / chin / beard | 6–10 wk | 65–70% |
| Underarm / axilla | ~3 mo | ~30% |
| Bikini / pubic | ~3 mo | ~30% |
| Chest / back / abdomen | 3–4 mo | 20–30% |
| Arms | ~18 wk | ~20% |
| Legs | 4–6 mo | ~20% |
We can clearly see that there is a large variation of timings and anagen percentages across the body. This means that we need to apply different intervals to different body sites, to achieve the optimum results.
Look at legs – only around 20% of the follicles are in anagen at any one time. Plus their telogen duration lasts for between four and six months. This means that most of the time when you’re treating legs, you’re probably not hitting very many anagen follicles at all – unless you leave a suitably long time between sessions.
How do we know when the best time is?
Well, there are three rules to consider:
- Intervals should be shorter than telogen. Follicle populations are asynchronous, so anagen entry is a rolling trickle rather than a batch arrival. Treating at roughly half to two-thirds of telogen catches each fresh cohort while the shaft is still short, pigmented and relatively shallow.
- Lengthen the interval as the course progresses. After three or four sessions the survivors are fewer, finer and slower to re-enter anagen. Sticking to a 4-weekly face protocol at session 6 is selling empty appointments! Not much will happen in these sessions!
- Regrowth, not the calendar, is the trigger. Teach them to distinguish exogen shedding at 1–3 weeks — which is the treated hair leaving, not new growth — from genuine regrowth. If nothing has come back, the session has nothing to target. This usually takes longer as the treatment programme progresses.
| Site | Sessions 1–4 | Sessions 5+ |
| Lip / chin / beard | 4 – 5 | 7 – 8 (don’t exceed ~10) |
| Axilla / underarms | 6 – 7 | 9 – 11 |
| Bikini / pubic | 6 – 7 | 9 – 11 |
| Chest / back / abdomen | 8 – 10 | 12 – 15 |
| Arms | 9 – 11 | 13 – 15 |
| Legs | 11 – 13 | 16 – 19 |
| Treat on visible regrowth, not on an empty area. Visible hairs mean full melanin and maximum matrix diameter — essential for the best results. | ||
| Lengthen the interval as the course progresses – you will catch more anagen follicles in the right state! | ||
| A missed appointment costs more than a delay if the follicle population has been partly synchronised — you lose the ‘phase’, not just the week. | ||
What about synchronised growth?
We know that delivering insufficient fluence to the follicles results in the anagen follicles beiong ‘forced’ into a ‘catagen-like’ state. Conseqeuntly, after a number of sessions, typically around 6 to 8, many follicles are growing, in the anagen phase, at the same time! This is the synchronised growth we often see when patients/clients say “all my hair is growing back!!”
It’s not that ALL of their hair is growing back – it’s nearly ALL of their remaining, viable follicles are growing simultaneously! These are the follicles which were not properly killed previously. (Usually due to too low a fluence…)
Summary
This has always been a tricky topic. Since follicles grow asynchronously and at different rates across the body, it is difficult to put hard and fast numbers on to this question. The table above gives an indication of the sorts of intervals we should be adopting. However there are bound to be some differences from person to person and various other reasons.
As I keep saying, the most important parameter when it comes to laser/IPL hair removal is fluence. If the fluence is too low (i.e. below threshold), then you will inevitably create a lot of catagen-like follicles. This appears to happen quite a lot out there as I see many reports of “all my hair is growing back“. When I see that I immediately think: “Your fluence was too low.“
I hope this helps,
Mike.
PS We’ll be talking more about this topic in an upcoming podcast episode…
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