Everyone is familiar with botulinum toxin type A, which has been available for cosmetic use in the form of Botox since 2002. The product is a powerful neurotoxin that paralyses muscles by blocking acetylcholine release at nerve terminals. In common with all neuromuscular blockers, it also has side effects. The most serious is that repeated treatment may cause muscle weakness, due to the destruction of motor neurons, more commonly known as ‘dying back’.
When Botox was first marketed for cosmetic use there were concerns from some quarters that it could eventually stop working – that prolonged use might lead to irreversible paralysis. There have even been reports of patients developing bulbar palsy after repeated treatment. However, most experts believe that the toxin does not cause progressive paralysis when applied in tiny doses for cosmetic use and they claim there is no evidence of it causing long-term damage to motor neurons or any other neuromuscular structure.
It has also been suggested that Botox may cause other, less serious, long-term side effects. These are more likely to occur in patients with abnormal neuromuscular function before treatment than healthy individuals. They include:
Drooping eyelids – this is caused by weakness of the muscle that raises the upper lid. The condition is sometimes reversible following botulinum toxin treatment.
Dysphagia (difficulty swallowing) – this may be caused by weakness of the muscles at the base of the tongue. It is very rare and is usually reversible.
Muscle weakness – which typically affects only a single muscle group, such as a hand or foot. The condition cannot be reversed but it is not generally permanent.
Muscle twitching – this is caused by the release of acetylcholine after muscle paralysis by botulinum toxin wears off, and is a sign that nerve terminals are recovering. It usually disappears within 3 months or so.
Although there have been no long-term studies on healthy individuals, cosmetic use of botulinum toxin is not thought to be dangerous provided that it is used once or twice a year and treatments are spaced at least 3 months apart.