What is the ‘best’ interval between hair removal sessions (after treatment by lasers or IPLs)? Part 2.

I wrote a blog post back in September 2021 discussing my thoughts on the how to determine the optimum interval between laser/IPL hair sessions. It proved very popular amongst my readers.

But, as soon as I posted it I began to doubt my own conclusions. Back then I suggested that we should wait for an interval equal to the anagen phase of the hairs being treated. This was based on the idea that we should always try to treat as many anagen follicles as possible, every time (since all light-based treatments destroy only the anagen follicles).

Table 1: Data from Richards-Merhag and my own calculations

This means we need to wait until the time when most of the follicles are back in anagen. Unfortunately, we can never know, in reality, at what stage the follicles are in! It’s impossible to tell!!

So, I created a wee model using Excel to investigate this. I used the total duration of anagen-catagen-telogen data and constructed a spreadsheet for a number of body areas including the upper lip, chin, arm pits, arms, bikini and legs.

A ‘set’ of hairs representing the upper lip, with an 8 week anagen phase and a 6 week telogen phase. The pink boxes are the anagen phase hairs, beige boxes are catagen and the green boxes are the telogen phase hairs. The time axis is pointing downwards (which is a bit odd – I know!!)

Now, this is where I noticed the first interesting thing…. The above 28 hairs represent EVERY hair in the upper lip area because every hair in that region will be in one of these stages in its growth cycle (assuming a 20 week anagen phase). So, simply by adding the durations of each phase, we can find out how many hairs are needed to represent all of them (using a 1 week temporal resolution)!

I coloured coded each phase so it is easy to see where the follicles are at each stage (see below). Next, I “killed” some follicles by turning them black – these were the follicles that were in anagen at the time of ‘treatment’ – week 0.

The black boxes represent all the dead follicles following treatment. In this case, these follicles were treated at week 0.

The image above shows the ‘live’ and ‘dead’ follicles after one treatment at week 0. In this case, 12 out of a total of 28 follicles have been killed. This is nearly 43% of all follicles.

If we look down the time axis (number of weeks on the left hand side), it becomes very obvious that the next ‘best’ time to treat is at week 8 – at this time all the remaining follicles are back in anagen!

Figure 3: By treating the live follicles at week 8, we can kill them all.

So, in this situation, we can successfully kill ALL the follicles in just two sessions…


And it’s a big but…..

This assumes a 100% efficiency in the treatment. This is very unlikely to occur in the real world.

In the real world, most people are using too low an energy (fluence) or too short a pulse (either because they have not set their equipment properly or it is not sufficiently good quality), or they have not treated the full area or some follicles are dormant. Or all of these!!

There are probably more reasons why 100% efficiency is virtually impossible to hit but I’m watching Shrek as I’m writing this and can’t think of any more…

So, I repeated the above tests using an 80% and then a 60% efficiency – much more realistic in the real world. I call this the ‘effective kill rate’ or ‘EKR’.

The table below shows what percentage of follicles may be killed in various body areas using three EKRs – 100%, 80% and 60%. It clearly indicates that the number of repeat sessions depends greatly on the EKR – obviously!

Table 2 – Each session will kill a certain percentage of anagen follicles, depending on the EKR. This table shows the amount of follicles killed in each session.

So, for instance, let’s look at the upper lip. All the hairs can be killed in just two sessions, if a perfect 100% efficiency is achieved (which is most unlikely).

But, an 80% efficiency can achieve the same result after just four sessions. The first session will kill 46% of the hairs, while the second will kill 36% – obviously, after each treatment there are fewer follicles to kill every time! It’s a case of ‘diminishing returns’ at each treatment. The last two sessions will kill the final 18% of live follicles.

But, when we look at the 60% efficiency, we see that this requires a total of 6 sessions to achieve the end goal. A lower efficiency will require even more sessions.

Remember, only anagen follicles can be killed by using light energy. But the ‘best’ results occur when there are the maximum number of anagen follicles at the treatment time – the table above assumes this condition. If the maximum number of anagen follicles are not present at the time of treatment then the overall efficiency will fall, resulting in more repeat sessions.


Treating hair follicles with light is quite tricky. You need to choose the correct wavelength(s), fluence and pulsewidths to ensure success. But, it also depends on the growth cycles of the target hairs – which vary enormously across the body according to table 1.

The number of treatments that operators need to achieve the result is indicative of how ‘efficient’ they are. If they need only five or six, then there are around 60% efficient – if they need more sessions, then their efficiency is lower. This means that they either need new, better equipment, or better training – or both!!

I am continuing to study these processes and will post up new results as I find them. I am currently looking at what effects occur when you change the interval times between four and 12 weeks….

Hope this helps,


2 thoughts on “What is the ‘best’ interval between hair removal sessions (after treatment by lasers or IPLs)? Part 2.

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