Background: Most research on medication adherence has focused on secondary nonadherence and persistence to therapy. Medication prescriptions that are never filled by patients (primary nonadherence) remain understudied in the general population. Methods: We linked prescribing data from primary care electronic medical records to comprehensive pharmacy dispensing claims between January 2013 and April 2019 in British Columbia (BC) to estimate primary nonadherence, defined as failure to dispense a new medication or its equivalent within 6 months of the prescription date. We used hierarchical multivariable logistic regression to determine prescriber, patient and medication factors associated with primary nonadherence among community-dwelling patients in primary care. Results: Among 150 565 new prescriptions to 34 243 patients, 17% of prescriptions were never filled. Primary nonadherence was highest for drugs prescribed mostly on an as-needed basis, including topical corticosteroids (35.1%) and antihistamines (23.4%). In multivariable analysis, primary nonadherence was lower for prescriptions issued by male prescribers (odds ratio [OR] 0.66, 95% confidence interval [CI] 0.50–0.88). Primary nonadherence decreased with patient age (OR 0.91, 95% CI 0.90–0.92 for each additional 10 years) but increased with polypharmacy among patients aged 65 years or older. Patients filled more than 82% of their medication prescriptions within 2 weeks after their primary care provider visit. Interpretation: The prevalence of primary nonadherence to new prescriptions was 17%. Interventions to address primary nonadherence could target older patients with multiple medication use and within the first 2 weeks of the prescription issue date.