Phototoxic and photosensitising substances

Knowing photoallergic and phototoxic substances present in cosmetic products, medication, and food is very important and helps avoid mistakes that might be upsetting for your skin. You can find a short description of the difference between these two groups below. You can also learn about substances which can be dangerous when paired with sun exposure or used during a series of laser treatments. Enjoy!

For sensitivity to occur, three factors need to be present:

  1. Photosensitive substance,
  2. Chemical substance,
  3. UV radiation

Photosensitive substances are chemical compounds with the ability to absorb UV radiation energy and causing toxic (phototoxic substances) or immunological (photosensitizing/photoallergic substances) reactions in the body. Phototoxic reactions are much more common than photoallergic response.

Phototoxic reaction

A phototoxic substance, once it comes into contact with UV radiation, causes erythema, oedema/swelling, a burning sensation, and, in severe cases, blisters. The clinical picture often resembles a sunburn, the reaction occurs in a rapid and acute manner. After the phototoxic substance is removed, the reaction gradually goes away. The changes are confined to the area of the skin that has been exposed to the radiation. UVB light is the type of radiation most commonly linked with phototoxic reactions.

Long term phototoxic reaction may cause skin discolouration.

Photosensitising reaction

Unlike phototoxic responses, photoallergic reactions have immunological causes. Photosensitising substances change their structure under the influence of UV radiation and cause the body to treat them as allergens. The immune system causes an allergic reaction which manifests as skin inflammation resembling eczema. One of its most acute symptoms is severe itchiness. In a photoallergic reaction, the helper cells and macrophages produce cytokines which destroy tissue. Inflammatory changes may also occur in places which were not exposed to the UV radiation (covered skin), and their symptoms remain even after removing the photoallergic substance from the body.

Common photosensitising substances include:

Substances derived from plants: 

Phototoxic substances:

  • St. John’s wort
  • Tar
  • Furanocoumarin (psoralen appearing in plants such as
  • Bishop’s weed, celery, carrot, Sosnowsky’s hogweed,
  • parsnips, dill, and parsley)
  • Citrus fruit rinds
  • Essential oils (bergamot, cedar, sandal, and citrus)

Photosensitising substances:

  • Mountain arnica
  • Chamomile
  • Pot marigold
  • Chrysanthemum
  • Yarrow
  • Dahlia
  • Tansy
  • Goldenrod
  • Chicory

Phototoxic substances:

  • Antibiotics (Tetracycline,
  • Sulfonamide, Quinolone)
  • Non-steroidal anti-inflammatory medication
  • (Ibuprofen, Ketoprofen, Naproxen)
  • Diuretics (Furosemide, Hydrochlorotiazide)
  • Diabetes medication (Sulfonylurea derivatives)
  • Photosensitisers 
  • Neuroleptics like Chlorpromazine
  • Retinoids 
  • Anti-fungal medication
  • Antiarrhythmic medication
  • Antiepileptic medication

Photosensitising substances:

  • Ketoprofen
  • Hydrochlorotiaziyd
  • Sulfonylurea derivatives
  • Chemotherapy drugs

Other photosensitising substances are:

Sun filters:
  • PABA
  • Oxybenzone
  • Cyclohexanol
  • Benzophenones
  • Salicylates
  • Cinnamate
  • Musk ambrette
  • 6- methylocumarin

Not every person using photosensitising substances experiences a photosensitive reaction. It is, however, essential to have the knowledge which could help avoid unpleasant side effects, especially when considering laser treatments or sunny holidays.

It is also important to know that photosensitising substances do not necessarily harm the skin. Quite contrary! Some of them can have a positive effect on the body, amongst them are sought after active ingredients. Their negative side effects only manifest when exposed to UV radiation. This is why it is imperative to use UVA and UVB filters while undergoing treatments containing phototoxic substances. Many active photosensitising substances should only be used at night, and many treatments only done in the Autumn/Winter months!

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