Many Rxs call for an "off-label" route
Many Rxs call for an "off-label" route to minimize side effects...cut costs...or because the desired dosage form or strength is not commercially available.
For example, pilocarpine eye drops can be given orally for dry mouth. This is a lot cheaper and works as well as Salagen tablets.
Atropine eye drops and Atrovent nasal spray can be given sublingually for drooling...with fewer systemic effects than with oral anticholinergics.