Introduction:
The aim of the study was to determine the relationship between medication adherence (MA) and selected psychological factors in a group of patients with rheumatoid arthritis (RA).

Material and methods:
The cross-sectional study was conducted in four rheumatology outpatient clinics in Silesia, Poland. The tests used were the Medication Adherence Questionnaire (MAQ), the Multidimensional Health Locus of Control Scale (MHLC), the Coping Inventory for Stressful Situations (CISS), and the Mindful Attention Awareness Scale (MAAS). The analysis involved 106 adult patients diagnosed with RA at least 6 months before, who were prescribed medication, with disease at any stage and with stable comorbidities. Software was used to perform analyses of frequency, basic descriptive statistics, including the Kolmogorov-Smirnov test, Student’s t-test for independent samples, intergroup univariate variance, Pearson’s r correlation coefficient, Spearman’s rank correlation  coefficient, Fisher’s exact test and stepwise linear regression.

Results:
Powerful Others Health Locus of Control (PHLC), Internal Health Locus of Control (IHLC) and age of the subjects, F(3, 102) = 8.05; p < 0.001 explained 16.8% of the variation in the adherence level for the entire group. In the group of women PHLC and IHLC, F(2, 80) = 10.04; p < 0.001 were included in the model, which explained 18.1% of variation in MA. PHLC was the most significant factor in the group of women ( = 0.55; p < 0.001) and in the entire group ( = 0.48; p < 0.001). In the group of men, Social Diversion Style (SDS), F(1, 21) = 5.81; p = 0.02 was included in the model, which explained 17.9% of the variation in the MA level.

Conclusions:
The study identified some psychological predictors of adherence, which explained 16.8% of the variability. Factors increasing the likelihood of medication adherence in patients with rheumatoid arthritis include a strong belief in the power of others, low level of internal health locus of control, and advanced age.