Final answer:
The drug of choice for treating decreased salivation accompanying head and neck irradiation is pilocarpine.
Step-by-step explanation:
The drug of choice for treating decreased salivation accompanying head and neck irradiation is pilocarpine. Pilocarpine is a nonspecific muscarinic agonist commonly used to treat disorders of the eye, but it also stimulates salivation. By constricting the pupil and causing the ciliary muscle to contract, pilocarpine opens perforations at the base of the cornea, allowing for the drainage of aqueous humor and reducing intraocular pressure. Thus, it can help alleviate decreased salivation in patients undergoing head and neck irradiation.