Statins are effective treatment for the prevention of cardiovascular diseases and used extensively worldwide. However, adverse effects induced by statins are the major barrier of maximalizing cardiovascular risk reduction. Hypothyroidism and administration of drugs metabolized on the same cytochrome P450 (CYPP450) pathways where statin biotransformation occurs represent a significant risk factor for statin induced adverse effects including myopathy. Simvastatin, atorvastatin and lovastatin are metabolized by CYP3A4, fluvastatin by CYP2C9, while rosuvastatin by CYP2C9 and 2C19. We investigated the levels of the free thyroid hormones and CYP metabolism of concomitant medication in 101 hyperlipidemic patients (age 61.3 +/- 9.9 ys) with statin induced adverse effects including myopathy (56 cases; 55.4%), hepatopathy (39 cases; 38.6%) and gastrointestinal adverse effects (24 cases; 23.8%). Abnormal thyroid hormone levels were found in 5 patients (4.95%); clinical hypothyroidism in 2 and hyperthyroidism in 3 cases. 11 patients had a positive history for hypothyroidism (10.9%). Myopathy occured in one patient with hypothyroidism and two patients with hyperthyroidism. There were no significant differences in the TSH, fT4 and fT3 levels between patients with statin induced myopathy and patients with other types of adverse effects. 78 patients (77.2%) were administered drugs metabolized by CYP isoforms also used by statins (3A4: 66 cases (65.3%); 2C9: 67 cases (66.3%); 2C19: 54 cases (53.5%)). Patients with myopathy took significantly more drugs metabolized by CYP3A4 compared to patients with other types of adverse effects (p < 0.05). More myopathy cases were found in patients on simvastatin treatment (52% vs. 38%, ns.), while significantly less patients with myopathy were on fluvastatin treatment (13% vs. 33%, p < 0.05) compared to patients with other types of statin induced adverse effects. Both abnormal thyroid hormone status and administration of drugs metabolized by CYP3A4, 2C9 and 2C19 are common in our patients with statin induced adverse effects. Normalizing the thyroid hormone status and optimizing of the concomitant medication may reduce the risk of statin induced adverse effects.