Thiomers are able to reversibly open tight junctions. The responsible mechanism seems to be based on the inhibition of protein tyrosine phosphatase being involved in the closing process of tight junctions. Due to thiolation the permeation enhancing effect of polymers such as polyacrylic acid or chitosan can be up to 10-fold improved. In comparison to most low molecular weight permeation enhancers, thiolated polymers offer the advantage of not being absorbed from the mucosal membrane. Hence, their permeation enhancing effect can be maintained for a comparatively longer period of time and systemic toxic side effects of the auxiliary agent can be excluded.