Purpose: Patients' adherence with statins pharmacotherapy is widely considered to be an important goal in the treatment of dyslipidemia. This study evaluates factors affecting patients' decisions regarding adherence with drug regimes.

Methods: We evaluated patients' adherence with prescription regimes by comparing between prescriptions given by physicians and actual purchases in a pharmacy by the patients. The prescriptions were given by general practitioners in primary care clinics to patients diagnosed with dyslipidemia. We analyzed data of 112,921 prescriptions of lipid-lowering drugs (statins), given by 195 physicians to 9,341 patients in 82 primary clinics between September 2002 and March 2006. Patients' adherence was measured by the probability of patients purchasing the prescribed drugs in pharmacies within 1 month after a prescription was given. We used a logistic regression to predict the patients' adherence with the prescriptions as a function of drug potency, co-payment, the total number of prescriptions given in the visit (apart from statins), co-morbidity (by Charlson Index), familiarity with the prescribing physician and patient's demographics of gender, age, and socio-economic status. This analysis is a part of a steady effort of a large-scale HMO to improve patients' adherence with dyslipidemia treatments.

Results: The average adherence rate was 79.7% and was stable across the different drug potency levels. Higher adherence was associated with older age, higher co-morbidity, and greater acquaintance with the prescribing physician. Lower adherence was associated with lower socio-economic status, male gender and higher total number of prescriptions given in the visit.

Conclusions: Patients' decisions regrading drugs utilization were affected by characteristics of the patient and the prescriptions, and the patient-physician relationship. Older patients, patients with higher co-morbidity, and those more familiar with the prescribing physician tended to adhere with the prescription regimes, while patients of a lower socio-economic status, males and those who received many prescriptions at each visit tended to adhere less with the prescription regimes. Patients, when deciding which drugs to purchase, apparently make a naïve cost-benefit analysis and purchase drugs according to this prioritization, which may lead to biased decisions due to lack of information about the effectiveness of the drugs. These findings can be used to predict patients' tendency to adhere to prescription regimes and possibly serve to implement interventions to improve adherence.