Having an injection of a very small volume of BoNT is not risk free, and patients may experience muscle tenderness or pain, weakness, or a general sense of being unwell. A few have anaphylactic reactions and there are reports that some have died.
"It's always important to look carefully at evidence from clinical trials to see whether a treatment is more effective than a placebo, and this is particularly important when treatments have known adverse effects," says the study's spokesperson, Dr Paul Michael Peloso, who works as a director of clinical research at Merck, in New Jersey, USA.
Some of the trials specifically compared the effects of either giving the toxin or a placebo injection of saline to two different groups of people with neck pain. The researchers could see no difference between the two groups either at 4 weeks or 6 months. Similarly adding BoNT to physiotherapy was no more effective than adding either an anaesthetic or saline.
"It is possible that BoNT did give some benefits that were not measured in the trials, or that it could help highly particular types of neck pain, but we would need some much more carefully conducted trials to reveal this," says Peloso.
"Based on current evidence we have no reason for supporting the use of BoNT as a stand-alone therapy for neck pain, but we do suggest that researchers consider further study to clarify whether the dose can be optimized for neck pain," says Peloso. The researchers also believe that trials should be run that look at other symptoms than pain, such as function, to see if there is any reason for believing that BoNT can provide some benefit.