Final answer:
The common side effect of phenytoin on mouth exam is gingival hyperplasia, which is the overgrowth of gum tissue.
Step-by-step explanation:
The common side effect of phenytoin (an anticonvulsant) seen on mouth exam is called gingival hyperplasia, which is the overgrowth of gum tissue around the teeth. Patients taking phenytoin may notice that their gums are swollen and that this overgrowth may even cover the teeth in severe cases. This side effect is important to recognize and manage with good oral hygiene and, in some cases, surgical intervention.