Thimerosal, or thiomersal, or merthiolate (sodiumethyl-mercurithiosalicylate) has been used as a preservative in vaccines and topical medication for years.
It is bacteriostatic against Gram+ and – bacteria, and active against fungi and yeast.
1 to 25% of positive patch-tests to thimerosal in patients with contact allergy.
10% of patients with positive patch-tests to thimerosal show adverse reactions to thimerosal containing vaccines.
Risk factors
Young adults (greater exposure to vaccines containing thimerosal?)
Clinical manifestations
Possible sources of thimerosal (0.001 to 0.1%).
  • Vaccines: diphtheria, tetanus, pertussis, mumps, hepatitis B, influenza, tick-borne encephalitis, staphylococcus, sal monella, meningococcus A.
  • Immunoglobulins: RhoD for example.
  • Extracts and diluents for: intracutaneous allergy tests, intracutaneous testing for candida, coccidioidin, histoplas min, mumps, hyposensitization therapy
  • Blood and plasma products
  • Topical medications: eyes, ENT area, skin
  • Storing and cleaning solutions for soft contact lenses.
  • Disinfectant for skin and mucous membranes
  • Cosmetic creams and lotions, toothpastes, mouthwashes, pesticides.

Persistent local reactions to vaccines.

  • Generalized urticaria, generalized exanthematic eruptions.
  • Asthma (one case).
  • Contact dermatitis.
  • Contact urticaria.
  • Keratoconjunctivitis (contact lens wearers).
  • Acute laryngeal obstruction (throat spray).
  • Prolonged external otitis (topical ear treatment).
Diagnostic methods
Patch-tests with: thimerosal 0.05% in pet. (0.1% is irritant)
thiosalicylic acid 0.1% in pet.
ethylmercurychloride 0.05% in pet
3 groups of patients are to be considered
  • positive to thimerosal, but negative to mercurials and thiosalicylic acid
  • positive to thimerosal and thiosalicylic acid, but negative to other mercurials
  • positive to thimerosal and some other mercurials, but negative to thiosalicylic acid.
The ethylmercury radical appears to be the allergenic determinant.
The high frequency of patch-test reactions to thimerosal is due to sensitization by thimerosal containing vaccines.
There is a cross-reactivity between thiosalicylate and a degraded photoproduct of
(sensitization to thimerosal with photosensitivity to piroxicam).
Hypersensitivity to thimerosal does not imply true mercury allergy. A positive patch-test with thimerosal should often be regarded as an accidental finding with no clinical relevance.
A history of ocular sensitivity to thimerosal does not preclude hepatitis B vaccine administration.
Replace thimerosal in soft contact lenses care with sterile single-unit preservative-free saline with thermal disinfection or use special preservative-free care system containing only a low concentration (0.6%) of hydrogen peroxide.


  1. Luka R.E, Oppenheimer J.J, Miller N, Rossi J, Bielory L, "Delayed hypersensitivity to thimerosal in Rho (D) immunoglobulin", J. Allergy. Clin. Immunol., 1997; 100 (1): 138-9 
  2. Gonçalo M, Figueiredo A, Gonçalo S, "Hypersensitivity to thimerosal: the sensitizing moiety", Contact. Dermatitis, 1996; 34: 201-3 
  3. van’tVeen A.J, van Joost T, "Sensitization to thimerosal (merthiolate) is still present today", Contact. Dermatitis, 1994; 31 (5): 293-8 
  4. Cirne de Castro J.L, Freitas J.P, Menezes-Brandao F, Themido R, "Sensitivity to thimerosal and photosensitivity to piroxicam", Contact. Dermatitis, 1991; 24 (3): 187-92 
  5. Aberer W, "Vaccination despite thimerosal sensitivity", Contact. Dermatitis, 1991; 24: 6-10

As in all diagnostic testing, the diagnosis is made by the physican based on both test results and the patient history.