Propylene Glycol -How Bad is it?

I was enjoying myself on Twitter the other day when I came across a company saying that propylene glycol was a bad thing because it modified the skin allowing toxins to penetrate it.  This sounded a bit scary so I decided to look into it. My first step was to ask them what research their claim was based on.  They didn’t answer — so I have investigated it myself

Propylene glycol seems to have acquired a very poor reputation.  Now, I have to say that I am not a fan of propylene glycol.  Its humectant properties stop the product from drying out too quickly (a humectant is simply a molecule that tends to hold water to itself).  Including a humectant can also help keep moisture in the skin.  We all benefit from keeping our skin moisture levels high.  Propylene glycol works pretty cost effectively -it is cheap – for both these purposes.

Why don’t I like it?  Well I would use it if I had too.  But I think that glycerol, which is a natural component of the skin, is a better choice.  I don’t see any good reason to add a foreign molecule to your skin when a perfectly natural one will do.  The extra cost does not have a huge impact on the final price of the product.

But I do remember once working on a project where we wanted to get something into the skin. We tried a range of things to help skin penetration, propylene glycol was one of them.  It didn’t work very well.  Nothing did in fact, I was left very impressed with how efficient the skin is as a barrier. But nonetheless propylene glycol has been used successfully to enhance penetration across the skin, so I asked the experts on the Skin Forum on LinkedIn.

It was an interesting discussion, and it seemed that some people have had some success at using propylene glycol though the consensus was that you needed quite a high level and that high levels cause irritation.  Once you get to 10%, then it is too irritating to use.  It was a really interesting discussion from which I learned a lot. Gerry Castling from the  University of Cincinnati’s observations were that propylene glycol can work to enhance penetration in combination with other penetration enhancers, but neat propylene glycol can slow down penetration, probably because it dries the skin out. David Enscore’s experience was much the same as mine: he hadn’t found propylene glycol to be a particularly good permeation enhancer. Daniel Bucks of Dow Pharmaceutical Sciences says that it works well in combination with ethanol but is too irritating above 10%.

Even better, Majela Lane sent me a paper hot off the presses – it was published on July 31st of this year which went into a lot of exactly the detail in which I had become interested.  This is a very detailed and technical paper with lots of numbers.  This is fantastic information if you are developing a formulation: it is like the difference between knowing that it costs a lot to fly to Manchester, and knowing the price of the ticket for the day you need to be there.   It also gives you a good feel for the way things work.  The actual work they had done centered around looking at the way propylene glycol affected the rate at which ibuprofen penetrated into the skin.

One quite simple fact sprung out at me.  They found that to get any serious increase in the rate at which the drug got through the skin, you needed to get up to 25% propylene glycol – and even that was in artificial skin that is easier to penetrate.  Not many cosmetic products would contain anything like 25% propylene glycol.  A typical level would be below 5%.  It was also very obvious that the effect on penetration wasn’t really all that great in any case, and the pattern of the data suggested that the effect was due to making the drug more soluble rather than affecting the permeability of the skin.  This study was not strong evidence that propylene glycol affects the skin’s structure at all, not even temporarily.

So what is my final conclusion?  It is far fetched to say that the low levels of propylene glycol that are used in cosmetic products are going to open your skin to toxins.  The skin is a good barrier and can easily cope with much worse stress than this.  On the other hand, propylene glycol isn’t as good a humectant as glycerol and if you have a choice of two products, one with propylene glycol and one with glycerol, the chances are the glycerol one will be better.

My thanks to the people who contributed to the discussion on the Skin Forum.

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References

Cooper, J Pharm Sci 73:1153-1156 (1984)

Aungst, Pharm Res 6:244-247 (1989)

Kasting et al. J Pharm Sci 82:551-552 (1993)

Skin Pharmacol Physiol 2009;22:225–230 Optimisation of Cosolvent Concentration for Topical Drug Delivery – II: Influence of Propylene Glycol on Ibuprofen Permeation R.M. Watkinson, R.H. Guy, J. Hadgraft,M.E. Lane

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