Which medication is associated with causing excessive hair growth as a side effect?

Phenytoin is known to cause hypertrichosis, which is the medical term for excessive hair growth. This side effect is particularly notable in patients who are on long-term therapy with phenytoin, often affecting areas such as the face and extremities. The exact mechanism is not completely understood, but it is believed to involve the drug's effects on hair follicle dynamics and the regulation of hair growth cycles.

Other medications listed, such as clonazepam, midazolam, and oxazepam, typically do not have hypertrichosis as a recognized side effect. Therefore, phenytoin is the medication that stands out for its association with this particular side effect, making it the correct choice in the context of the question.