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Treatment with low dose methotrexate in rheumatoid arthritis is associated with serious side effects in about 5 per cent of cases (respiratory, haematological or infectious). The goal of a null risk seems unrealistic because of the idiosyncrasy of some of the risks and our poor understanding of others (enzymatic polymorphisms might be operational, and infectious agents could act as co-factors). However, risk can be greatly reduced by a careful selection of patients. Some contraindications are strict: poor compliance and the possibility of mistake in the timing of the administration; pregnancy or desire for pregnancy; treatment with trimetoprim; haemodialysis; renal insufficiency (clearance < or = 50 ml/min) (and therefore old age), alcoholism. Others remain relative although well established; hypoalbuminaemia, diabetes mellitus, obesity, past infection with hepatitis virus. Others are dubious: starvation, macrocytosis, surgical stress, NSAIDs. An extensive large study of side effects is warranted.