According to research conducted by Harvard Medical School, not all botulinum toxins are safe for neurons. Lead researcher, Dr Min Dong, said that two out of seven toxins, type C and E, cause neuron death. The study was published in Nature Communications in February.
Botulinum toxin type A, which is used in Botox, is safe for healthy neurons however caused neuron death in a lab study when mutations are introduced.
It is already well established that botulinum toxins disable signals from neurons to muscles by attacking protein inside neurons. Type C and E toxins destroy the neuron's ability to recycle plasma membrane which is crucial for its survival.
Whilst Type A toxins do not generally cause harm, it's safety is not absolute. According to Dr. Dong, the death of neurons which were mutated is something that should be further explored, however its use in cosmetic procedures was safe because of the dosage.
Dr. Dong recommends that dermatologists practice caution when using types C and E to treat patients. Whilst these toxins are not yet used commercially, studies are being conducted which use type C toxins as a muscle relaxant.