Cyclopentasiloxane Safety Measures/Side Effects:
Cyclopentasiloxane may cause mild skin and eye irritation, according to the ingredient’s material safety data sheet, and a small percentage of it reaches systemic circulation through dermal absorption, as found in a study by the Department of Environmental and Radiological Health Sciences.
Dimethicone Safety Measures/Side Effects:
The Food and Drug Administration (FDA)has approved its use as a “skin protectant” ingredient in over-the-counter (OTC) drug products. Furthermore, the Cosmetic Ingredient review (CIR) Expert Panel reviewed the data surrounding dimethicone, and assessed it as safe to use in personal care products. They determined that because of the large molecular weight of silicone based polymers such as dimethicone, it would be very unlikely for it to penetrate past the surface of the skin.
Both are silicones, but as you can see only Dimethicone/Dimethicon is what we can call safe because can not be absorbed by skin.