Which of the following factors may limit the use of a prescription drug coupon?

The most relevant factor that can limit the use of a prescription drug coupon is the coupon's expiration date. Prescription drug coupons are often subject to time constraints; once they expire, they can no longer be used, which can restrict a patient’s ability to benefit from the coupon when they go to fill their prescription.

While a patient's age, the quantity of medication prescribed, and the patient's type of insurance may present additional considerations in the overall context of pharmacy billing and reimbursement, they do not directly relate to the fundamental aspects of how a coupon operates. The expiration date is a clear and definitive limitation associated with any promotional offering, including drug coupons, making it crucial for patients to be aware of these details to effectively utilize their savings.