Psoriasis is relatively common in clinical practice, whereas niacin deficiency is relatively rare. We describe the clinical case of a patient with plaque psoriasis for over 20 years who also had a concomitant latent tuberculosis infection. After secukinumab and anti-tuberculosis treatment for 1 year, the psoriatic rash mostly resolved, but atypical symptoms of niacin deficiency suddenly appeared. The patient's symptoms rapidly subsided after experimental treatment with niacin. After 2 weeks, the patient suddenly developed an erythroderma-like rash, manifesting as large areas of erythema and plaque psoriasis throughout the body. The patient was admitted to the hospital and treated with an anti-inflammatory, biologic adalimumab, tripterygium glycoside, and sodium thiosulfate. The patient was discharged after a week. This case suggests the need for caution and to look out for the emergence of new symptoms when treating patients with moderate-to-severe plaque psoriasis, especially with biologics.