Despite the positive effects of isotretinoin on a number of cancers and severe skin conditions, several disorders of the musculoskeletal system have been reported in patients who are treated with it. Reactive seronegative arthritis and sacroiliitis are very rare side effects1, 2, 3. We describe 4 cases of inflammatory back pain without sacroiliitis after a month of isotretinoin therapy. We observed that after termination of the isotretinoin therapy, patients’ complaints completely resolved. Musculoskeletal system side effects reported from isotretinoin treatment include skeletal hyperostosis, calcification of tendons and ligaments, premature epiphyseal closure, decreases in bone mineral density, back pain, myalgia and arthralgia, transient pain in the chest, arthritis, tendonitis, other types of bone abnormalities, elevations of creatine phosphokinase, and rare reports of rhabdomyolysis. Musculoskeletal symptoms are common with isotretinoin therapy, but sacroiliitis is a very rare adverse event considering the number of patients treated with these drugs. Case 1. A 32-year-old woman started isotretinoin for treatment of acne vulgaris. On the 15th day of treatment, severe backache, hip joint pain, and morning stiffness lasting 1–2 h occurred. Seronegative arthritis was suspected. Sacroiliac magnetic resonance imaging (MRI) was normal and HLA-B27 test was negative. Her erythrocyte sedimentation rate (ESR) was 16 mm/h and C-reactive protein (CRP) was 2 mg/l. Her complaints diminished with indomethacin and ibuprofen treatment. However, morning stiffness lasting half an hour and pains at night resumed. She discontinued the isotretinoin medication in the fifth month. After 20 days her complaints were resolved and never recurred.

Case 2. A 22-year-old man took 15 mg/day isotretinoin for treatment of acne vulgaris. On the 20th day, he complained about backache and hip joint pain that led to disrupted sleep. His complaints continued during the day. He had morning stiffness for 2 h. He was evaluated for seronegative arthritis. Sacroiliac MRI was normal and HLA-B27 test was negative. The ESR was 24 mm/h and CRP 3.4 mg/l. His complaints lessened with nonsteroidal antiinflammatory drug (NSAID) treatment. He discontinued isotretinoin before completing the 6-month treatment period. After 1 month his complaints resolved.