Scientists gave warned that botox injections - used by millions every year to banish wrinkles and plump-up lips - may cause muscles to waste away into fat.
Researchers from the University of Calgary have found that the cosmetic treatment could cause harmful side effects - even areas of the body which have not been injected.
They warn that the jab could cause harmful side effects when used over long periods of time.
For the first time experts have found that Botox causes muscle wastage in parts of the body not injected.
Researchers studied the effects of the substance, known amongst in the medical profession as Botulinum toxin A, on a group of 18 rabbits over a period of up to six months.
Their study found that limbs which had been injected with the substance experienced muscle wastage of up to 50 per cent.
But alarmingly they also discovered that muscle loss also occurred in limbs that hadn't been injected - suggesting the harmful effects of the jab could be spread to other parts of the body.
The researchers warn that their findings 'raise important questions' about the long-term use of the popular cosmetic treatment.
"What we have shown is some muscle tissue is being replaced by fat," the Daily Mail quoted lead author Rafael Fortuna as saying.
"Basically, we should be more cautious. We should be aware once we have Botox, there might be some side effects far removed from the injection site.
"We were surprised by the degree of muscle loss and atrophy in the limb that was not injected with the Botulinum toxin.
"I think it's fair to say that the paper raises some important questions about the long-term therapeutic use of Botox, especially with children and adolescents," Fortuna added.
The study has been published in the Journal of Biomechanics.