Photosensitive or Photosensitizer? Differences and Meaning

Let's start with the definitions:

  • A substance is photosensitive if it reacts to light, especially ultraviolet (UV) light, and can become more or less active, or undergo chemical changes when exposed to it.
  • A substance is photosensitizing when it can cause skin reactions, such as rashes or irritations, when the skin is exposed to light after the product has been applied.

A couple of examples

Two common examples in cosmetics are vitamin C and retinoids.

Vitamin C is photosensitive because it breaks down when exposed to sunlight (via an oxidation reaction). This does not mean the product will have negative effects on our skin, but the substance itself will definitely degrade. This is why it is usually packaged in an opaque tube that blocks air exposure, or in a dispenser pump package, to preserve the stability of the ingredients even after you start using the product. Stabilized formulations (such as 3-0-ethyl ascorbic acid) are also commonly used.

Retinoids , on the other hand, are photosensitizing and this is why it is best to apply retinol as part of your evening skincare routine, and always wear SPF the following morning to protect your skin from potential damage if you are exposed to sunlight.