What side effect might you expect from the use of botulinum toxin?

The use of botulinum toxin, commonly known for its application in treating various conditions like muscle spasms and for cosmetic purposes, is associated with a specific side effect known as dry mouth, or xerostomia. This occurs because botulinum toxin works by blocking the release of acetylcholine at the neuromuscular junction, which inhibits the activation of glands responsible for saliva production, leading to reduced salivation and consequently a dry mouth.

In the context of the other options, elevated heart rate is not typically associated with botulinum toxin use; in fact, it can cause muscle weakness that may lead to a lower heart rate. Increased appetite is also not a known side effect, as the toxin does not influence hunger or metabolic drive directly. Lastly, excessive urination isn't a result of botulinum toxin; it more typically relates to other medications or conditions affecting the urinary system. Therefore, dry mouth is the expected side effect when considering the mechanism of action and common clinical outcomes associated with botulinum toxin.