Insulin resistance is a predictive factor of response to treatment with peginterferon and ribavirin in patients with hepatitis C. Insulin resistance impairs sensitivity to interferon and can block its intracellular signalling. Insulin resistance also induces the development of steatosis, progression of fibrosis and proinflammatory cytokine release and reduces the bioavailability of interferon. Suppressor of cytokine signalling 3 and protein tyrosine phosphatases are involved in blocking the intracellular signalling of interferon and insulin. Insulin resistance can be treated through diet, physical exercise and the use of insulin-sensitizing agents such as biguanides or glitazones. The TRIC-1 study demonstrated that adding metformin to routine treatment improves the possibilities of cure in women and in patients whose insulin sensitivity returns to normal during treatment.