Choosing the ‘best’ time between laser/IPL hair removal sessions is not simple. It depends on the hair growth cycles of the hairs we are targeting, which is not the same across the body. That means that we need to consider the body area when deciding how long to leave between sessions.
We can only target follicles in the anagen phase, when using light-based treatments. So, to improve the results, we should only treat when most of the follicles are in that stage of the growth cycle.
The table below shows the ‘optimum’ interval to wait between sessions, based on the above criteria:
| Body site | 2nd treatment (weeks after 1st) | 3rd treatment (weeks after 2nd) |
| Chin | 4 | Any time |
| Upper Lip | 8 | 2 – 4 |
| Arm pits | 8 | 4 – 12 |
| Bikini | 7 | 6 – 10 |
| Arms | 12 | 6 – 8 |
| Legs | 8 | 14 – 16 |
The above figures are calculated using a model of the hair growth cycles in the skin, which I created using MS Excel. They assume that the hair owner is fit and healthy and has no underlying conditions which might affect the growth cycles.
Finding the interval between the first and second treatments is easy – it is quite obvious from the model. Thereafter, it becomes trickier because of the random nature of laser/IPL hair removal – this is why I have put a range of weeks, rather than a single number.
| Body Area | Duration of Anagen (weeks) | Duration of Telogen (weeks) | % Growing Hair Anagen | % Resting Hairs Telogen |
| Chin | 52 | 10 | 81 | 16 |
| Upper Lip | 8 – 20 | 6 | 43 – 71 | 43 – 21 |
| Arm Pits | 16 | 12 | 53 | 40 |
| Bikini | 16 | 12 | 53 | 40 |
| Arms | 12 | 16 | 40 | 53 |
| Legs & Thighs | 16 | 20 | 42 | 53 |
| Scalp | 2-6 years | 12 – 16 | – | – |
Table 2 shows the duration and percentages of the anagen and telogen phases in a number of body areas. It is based on studies from the 1970s (Richards and Merhag).
It is impossible to be 100% efficient with these techniques. Achieving an 80% efficiency (when you kill around 80% of the anagen follicles) is possible if you are using good equipment and are well trained.
However, my results (not shown here) appear to indicate that most users are achieving around a 60% efficiency. This means that they will inevitably require more repeat sessions to achieve a decent result. Using the numbers above should help to improve overall results in many cases.
In reality, it is very difficult to choose the ‘best’ interval after the first two or three treatments. I know that the intervals in Table 1 look “strange” and don’t appear to correlate with the duration of the anagen phases – that is because the optimum intervals are dependent on both the anagen and telogen durations – not simply the anagen duration as I stated in a previous blog post in Sept 2021!!
I am continuing my work on this since it is important for all you guys who perform this treatment. I will publish more results soon.
Hope this helps,
Mike.
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