Undertreatment is a related term of overtreatment.

In interior decoration|lang=en terms the difference between undertreatment and overtreatment

is that undertreatment is (interior decoration) a window treatment applied underneath an existing treatment while overtreatment is (interior decoration) a window treatment added above an existing treatment.

As nouns the difference between undertreatment and overtreatment

is that undertreatment is insufficient treatment, often specifically medical treatment while overtreatment is (medicine) excessive treatment, often specifically medical treatment.