CO 91 denial code

Claim adjustment reason code 91. Often reported as CO 91.

Dispensing fee adjusted

Unofficial reference. This is our own plain-English explanation, not the official code text. Payer rules differ. Verify with the payer and the official X12 code list. AlphaPrefix is not affiliated with X12, CMS or any payer. Data last updated Nov 11, 2026.

What 91 means

The payer adjusted the dispensing fee on a pharmacy claim to the amount it allows.

Pharmacy claims usually carry two parts: the drug cost and a fee for dispensing it. This code tells you the payer changed the fee portion, typically to the rate in its pharmacy agreement.

Fee differences usually come from the contract, the type of prescription, or the number of fills allowed in a period, not from the drug itself.

Group codes seen with 91

On a remittance, 91 comes after a group code. These are the ones it is most often reported with:

• CO 91 (Contractual Obligation): The provider's side of the ledger. The reduction comes from the payer contract or a rule the provider has to follow, and the patient normally cannot be billed for it.

The payer chooses the group code for each adjustment, so the same reason code can appear with another group code on your remittance. Read the two together. More about group codes.

Common causes

• The submitted dispensing fee was higher than the rate in the pharmacy agreement.
• The payer limits dispensing fees for repeat fills within a set period.
• The prescription type, such as a compound, falls under a different fee schedule than the one used on the claim.

What to do next

1. Check the dispensing fee rate in the pharmacy agreement for this claim type.
2. Confirm the fill count and days' supply did not trigger a fee limit.
3. Update the default fee submitted to this payer if it is consistently above the contract rate.

Related codes

• 90 (reason code) Drug ingredient cost adjusted
• 45 (reason code) Charge exceeds the allowed amount