['Emerging data suggest that mental illness and substance use disorder (SUD) are important risk factors for inconsistent contraceptive use. We investigated whether mental illness without or with SUD is associated with contraceptive adherence and continuation of hormonal methods among women Veterans.', "We conducted a retrospective analysis of national Veteran's Administration data among women aged 18-45 with a hormonal contraceptive prescription (pill/patch/ring/injectable) during the first week of fiscal year 2013. We tested associations between mental illness diagnoses (depression, posttraumatic stress disorder, anxiety, bipolar disorder, schizophrenia, adjustment disorder) without or with SUD diagnoses (drug/alcohol abuse) and 12-month contraceptive adherence (number and length of gaps ≥7 days between refills and months of contraceptive coverage) using multivariable regression models.", 'Among 9780 Veterans, 43.6% had mental illness alone, 9.4% comorbid mental illness and SUD, and 47.0% neither diagnosis. In adjusted analyses, compared to women with neither diagnosis, women with mental illness alone had a similar rate of gaps but increased odds of having gaps longer than 30 days [odds ratio (OR): 1.35, 95% confidence interval (CI): 1.10-1.52] and fewer months of contraceptive coverage (β_coefficient: -0.39, 95% CI: -0.56 to -0.23). Women with mental illness and SUD experienced more gaps (incidence rate ratio: 1.12, 95% CI: 1.03-1.21), increased odds of gaps longer than 30 days (OR: 1.46, 95% CI: 1.10-1.79), fewer months of contraceptive coverage (β_coefficient: -0.90, 95% CI: -1.20 to -0.62) and reduced odds of continuous 12-month coverage (adjusted OR: 0.76, 95% CI: 0.63-0.93).'] Do contraceptive adherence among women Veterans with mental illness and substance use disorder?