Shoppers Drug Mart is recalling some packages of Alesse 21 birth control that expired last year, after it mistakenly dispensed the contraceptive to customers in Western Canada.

Health Canada said efforts are being made to contact patients who may have used the oral contraceptive, warning that “products beyond their expiry may lead to reduced effectiveness.”

The agency said the recall affects the following packages of Alesse 21:

DIN: 02236974

02236974 Strength: 100 µg of levonorgestrel and 20µg ethinyl estradiol

100 µg of levonorgestrel and 20µg ethinyl estradiol Lot number: G73720

G73720 Expiration date: Sept. 2014

Patients who have the expired Alesse 21 are asked to return the affected product to their pharmacist for a replacement, speak to a healthcare practitioner if the expired product has been used, and report any adverse health effects to Health Canada. They can call the agency toll-free 1-866-234-2345

Shoppers Drug Mart said it dispensed the expired product in British Columbia, Alberta, Saskatchewan and Manitoba between March 16 and April 1.

The company that manufactures the birth control medication, Pfizer Canada, told The Canadian Press in a statement that the drug market chain "quickly" informed the company and "took immediate corrective measures."