Biologics such as tumor necrosis factor (TNF) inhibitors are highly effective for treating severe psoriasis, and an increasing number of patients benefit from them. However, subsidiary subjects have arisen in the course of treatment which may directly impact the continued use of these drugs in a given case. In this article, we report two cases of psoriasis vulgaris refractory to treatment with both adalimumab and infliximab. We measured the serum drug concentration and absorbance value of antidrug antibodies using the sandwich enzyme-linked immunoassay method. Antibodies to adalimumab and infliximab were detected in both patients. The low drug concentration and adverse clinical response observed corroborated these findings. Dosage and concomitant medication are considered to be important factors influencing the development of antibody formation. Therapeutic drug monitoring and measurement of antidrug antibodies will assist in maintaining a clinical response and in optimizing the response of patients being treated with biological therapies. The use of concomitant immunosuppressive medication should be considered for patients who will be treated with their second TNF inhibitor after having developed antidrug antibodies against their first TNF inhibitor.