A side effect is easier to judge when you know in advance what the sources call normal. This article lays out what the documented sources describe, so a reader in Ann Arbor, Michigan knows where normal ends and a phone call begins.
What the sources describe as normal
aad.org states the baseline: "If a patient develops side effects, these tend to be mild and temporary." For product background, dermnetnz.org has a page on botulinum toxin, Botox and Dysport.
Where most of the trouble starts
aad.org adds a detail about the cause: "The injections, rather than the botulinum toxin itself, cause most mild side effects." That makes the injection visit, and not only the product, something to ask about.
Follow-up after the visit
If you go back for repeat injections, each visit carries the same injection step the source links to most mild side effects. One research article looks at neurotoxin content and the potential implications for duration of response, which is useful reading when you plan how often to return. If something does not fit the mild and temporary description, that is the moment to contact the practice.
What to ask before you commit
Ask who you should call if an effect lingers, how soon you can be seen, and what a corrective visit costs.