In the United States, a formulary is the list of medications available to someone enrolled in a specific drug insurance program.
Usually a "tiered" system is in place as an incentive for patients to select lower-cost medications. Under a three tier system (the most common), first tier has the least expensive generic drugs and the lowest co-pay. As the medication cost increases, it is listed in a higher tier and thus has a higher co-pay. Usually the third tier has most brand-name medications and the highest co-pay.
The formularies are in place to help insurance manage drug costs imposed on the insurance policy. If the drug is not on the formulary, it usually requires a "prior authorization" -- meaning the insurance company wants to talk to the prescriber to see if there is a lower-cost alternative for the patient. Formularies vary from plan to plan and insurance to insurance. Co-pays vary as well.
Your pharmacy has no control over your insurance, your formulary, or your co-pay. As a courtesy we bill the insurance electronically in your behalf. Within seconds we receive information on if your claim was paid and the amount of your co-pay. Sometimes we get a reject saying "prior authorization" required or the medication your insurance will pay for.
You can always find out if a medication is on your formulary by calling your drug insurance company or looking on their website. Your pharmacist has no way of knowing what is on your formulary without a valid prescription to adjudicate.