I have read that SLS can be harmful. Is that true?

Sodium Lauryl Sulfate (SLS) is a widely used ingredient in shampoos. A distinction is made between SLES - Sodium Laureth Sulfate - and SLS, which tends to irritate the skin. This is a modified, milder form and is generally well tolerated. Thymuskin uses SLES in the Classic Shampoo because oily hair requires a stronger cleansing action. In the Sensitive Shampoo, it is only contained in very small quantities, combined with so-called organic surfactants and is therefore suitable for sensitive hair. The Forte Shampoo is completely SLES-free.