Laser/IPL Hair Removal – How many follicles do we kill each treatment?

All hair follicles grow according to a cycle – ‘anagen, catagen, telogen’. The follicle germative cells promote anagen hair growth at consistently regular intervals resulting in periods of active growth, followed by inactivity. The duration of these periods depends on the which part of the body is being considered.

I created a computer model to investigate these growth cycles. I wanted to determine the ‘best’ intervals between laser/IPL treatment sessions to generate the best outcomes. My main criteria was that we wanted to retreat areas when the maximum number of follicles were back in anagen.

Upper lipChinBikiniArm
Table 1: Number of sessions required to kill almost all follicles across the body

In the real world, nobody can effect a 100% efficient treatment. So, we can never kill every anagen follicle when we treat. In reality, the efficiency is somewhere between 60 and 80%, in most cases. Table 1 shows the number of sessions required to kill virtually all follicles across the body when assuming a 60% efficiency, for three intervals between sessions – 4, 6 and 8 weeks. The number of sessions required doesn’t change that much and these numbers are very close to what most clinics around the world achieve. 

An 80% efficiency will reduce those numbers by only 1 or 2 sessions! The two ways to improve efficiency are 1: better equipment and 2: better training. 

However, my model clearly shows that the first treatment will always generate the highest clearance – around 33%. Subsequent treatments kill fewer follicles, simply because there are less to treat each time.

Figure 1: Each treatment can kill a certain percentage of follicles, but fewer follicles are killed each time. The difference between 4 and 8 week intervals can be seen. Leaving an 8 week gap between treatments results in more clearance of the follicles.

Each time a treatment is carried out, fewer follicles are destroyed than previously (see table 2).

% follicles killed after
each session
4 weeks8 weeks
Table 2: The difference between leaving 4 or 8 weeks makes a slight difference in the % of follicles killed.

From the table above, it is clear that this treatment is a case of ‘diminishing returns’. Each treatment can kill a certain percentage of follicles, but every time a client/patient returns, there are fewer follicles to treat.

We can also see that leaving an 8 week gap between repeat treatments results in a little more clearance of follicles. But the difference is marginal! 


The gap between treatment sessions does not appear to be important! Contrary to my previous posts, “optimising” this gap makes a very small difference in reality. The difference between a short or a long gap will change the overall number of sessions required – but only by one or two sessions!

So, choose whichever gap suits you and your business!!

Hope this helps,


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