What it does
The US Cosmetic Ingredient Review defines dimethicone as a fluid mixture of fully methylated linear siloxane polymers, end-blocked with trimethylsiloxy units. The silicones in that review work mostly as conditioning agents, at reported concentrations of up to 15%.1 The EU's CosIng database lists four functions: skin conditioning, emollient, skin protecting and antifoaming.2
On hair it is the most widely used silicone and a main ingredient of two-in-one shampoos, where it protects the hair shaft from abrasion.3 Like other silicones it coats the strand, which reduces friction during combing and adds shine.4
On skin it acts as an emollient and occlusive, leaving a thin, non-greasy layer.5 In the US it is also one of the active ingredients permitted in over-the-counter skin protectant products, at 1% to 30%.6
Where you find it
In shampoos, conditioners, masks and serums, in moisturisers, and in makeup and sunscreens, where silicones help pigments spread and give a light, non-greasy feel.5
The EU cosmetics regulation has no annex entry of its own for dimethicone. It does name it as one of the coatings allowed on two nano-form UV filters, titanium dioxide and zinc oxide.7
Build-up, and what sources say
Dimethicone does not dissolve in water. A 2025 review notes that with prolonged use it can accumulate on the hair shaft, particularly on fine or oily hair, and that removing it takes stronger surfactants, such as sulfate clarifying shampoos, whose overuse may dry the hair.4 In a shampoo it can also reduce the amount of lather.5 Neither source puts a number on how often build-up happens.
The CIR panel found that dimethicone was not absorbed through the skin or the gut in clinical and animal studies, and it was not a sensitiser at 5% in a repeated patch test on 83 volunteers. The panel considered significant absorption into the skin unlikely because of the polymers' large molecular weight.1
How it appears on a label
As Dimethicone; chemists also call it polydimethylsiloxane.5 Many longer names are built on it, such as Cetyl Dimethicone or Stearyl Dimethicone, which the US panel assessed in the same review.1
EU claims guidance says a product that contains silicone should not be called silicone-free.8 For how dimethicone compares with amodimethicone and the volatile silicones, see silicones in hair products.