Introduction: Intramuscular (IM) injection of corticosteroids (CS) is recommended and routinely used to suppress inflammation and relieve pain during an acute episode or exacerbation of rheumatoid arthritis (RA). We present the case of a patient with RA who developed significant muscle loss following an IM CS (triamcinolone acetonide, 40 mg) injection given for a disease 'flare'. Case presentation: During a clinical trial assessing the effects of oral creatine monohydrate supplementation, we were able to assess changes to dual x-ray absorptiometry (DXA)-assessed body composition following an IM injection given to one of our patients. Although DAS28 was dramatically improved following the injection, assessment revealed a 2.4 kg (4%) loss of total lean mass (TLM) had occurred, the majority of which occurred in the arms and legs (2.0 kg (7%)). Discussion: Whilst it is possible that the depletion of LM occurred between the onset of flare and the CS injection, this single case report raises the possibility that the IM CS injection, which is regularly used and recommended as treatment of active RA, may be contributing to the substantial and sudden loss of muscle mass observed. Keywords: Rheumatoid arthritis, muscular atrophy, cachexia, intramuscular corticosteroid injection