The study was conducted by a team of German researchers who found that one in 200 patients who took Botox developed antibodies to the treatment, which meant that it became less effective in freezing their facial muscles over time.
The researchers also found that those who had been prescribed higher doses of Botox, for medical reasons to treat spasms or sweating, were also more likely to stop responding to the treatment.
The study was also confirmed by a number of British experts who wrote in BMJ's online forum Open that they found some of their regular patients not responding to the treatment. Says Dr Kuldeep Minocha, of Harley Street's Absolute Aesthetics Clinic, "With people who have been using Botox for ten years or so, you can start to see a bit more resistance. The results can wear out a bit quicker because their body seems to get used to it and recognizes it as a foreign body, and breaks it down quicker."