The injection site is not necessarily where the effect stays. That detail is easy to miss, and it matters most when a patient reports a problem after the visit.
A complaint far from the injection site
A patient who reports a problem far from the treated area may be describing the spread MedlinePlus warns about below. Readers who want background on the condition behind this toxin can start with MedlinePlus on botulism.
Spread to muscles elsewhere in the body
MedlinePlus puts the finding in one sentence: "However, it is possible that the medication may spread from the area of injection and affect muscles in other areas of the body."
The wording is careful. It says the medication may spread and affect muscles elsewhere. It does not say how often, so do not promise a patient a number.
The label and the product
For a general reference on the substance, Wikipedia's entry on botulinum toxin is a plain place to begin. For one specific product, MedlinePlus has drug information on prabotulinumtoxinA-xvfs injection.
Warn before, ask after
Tell every patient before the injection that effects away from the site are possible, and ask them to contact you if they notice any. Cleveland Clinic offers a patient-facing page on botulinum toxin injections to hand over alongside that conversation. If you train in or near Parsippany, New Jersey, put the follow-up call in your routine from the first patient.