Women who regularly get Botox treatments seeking youth are becoming immune to the injection, even forming antibodies to it, a report said.
German researchers published a paper in the Journal of Neural Transmission, saying that one out of 200 Botox patients come to develop antibodies which lead to ineffective results.
More negative results were found among those who took a larger dose of injection than others due to medical reasons such as treating spasms or sweating.
Other scientists from British Medical Journal's online forum also found that repeated treatment with Botox "triggers an immune response ... which might lead to non-responsiveness to treatment."
Dr. Kuldeep Minocha of Harley Street's Absolute Aesthetics Clinic also said his patients became immune to the treatment, particularly those who have gotten the injections for about 10 years. If the patients' bodies get used to the chemical, Botox gets broken down more quickly, resulting in less effects.
He said that while the reported number of antibodies formed remained relatively small, the actual figure might be much higher since few patients go through close examinations in order to find out the cause of failed treatments.