Those of us fortunate enough to have comprehensive health insurance tend to maximize the benefits whenever possible. It’s rare that patients are better off dealing with a medical provider or service on a cash basis than with their insurance card and co-pay. The exception? Certain prescription medication.

According to U.S. News & World Report, there are some instances where paying out of pocket for prescription drugs makes sense. Despite the common belief that a covered drug must be cheaper than if you were uninsured, that’s not always true. Some medications to control blood pressure or cholesterol, for example, are priced by manufacturers and pharmacies to be competitive at a retail price in the hopes it will attract consumers to come into the store and spend more on other necessities like toilet paper or shampoos. This strategy might be more commonplace at a large discount chain like Costco, where someone coming in to fill a prescription could spend hundreds elsewhere in the store.

If that $4 or $8 cash refill is less than your $15 co-pay, the pharmacist would let you know, right? Not necessarily. Although many insurance plans promise you’ll be charged the lowest price, in practice some pharmacists might be obligated to stick to the listed rate and prevented from disclosing the cheaper cash price by pharmacy benefit managers looking to maximize revenue. That’s why it can be beneficial to check a site like BlinkHealth that can alert you to cash prices for commonly-prescribed medications.

There is one caveat: If you pay cash for some of your prescriptions, the money spent won’t be applied toward your insurance plan's prescription deductibles.

[h/t U.S. News & World Report]