If the two companies cannot agree on reimbursement rates for drugs by the end of the year, those who carry Express Scripts will not be able to get their prescriptions filled at Walgreens.
With more than 7,700 drug stores nation-wide Walgreens spokesman Michael Polzin says it could be a hardship for some customers.
"We are in urban areas and some rural areas where there are not other pharmacies or if there are other pharmacies they have very limited hours versus our 24-hour access to a pharmacy and pharmacist, so I think access will definitely be an issue for some patients," said Polzin.
But Express Scripts says there is a drugstore within a half mile of most Walgreens.
And the St. Louis-based pharmacy benefits manager says it has agreements with all its employers and insurance providers to provide geographic access.
The companies' contract expires Saturday.