Antibody formation to ustekinumab has been observed with therapy and has been associated with decreased serum levels and therapeutic response in some patients. May increase the risk of infections and reactivation of latent infections. Should not be given to patients with any clinically important active infection (caution in patients with a chronic infection or history of recurrent infection). Patients taking ustekinumab have theoretically more risk for particular infections, and appropriate diagnostic testing should be considered (individuals genetically deficient in IL-12/IL-23 are particularly vulnerable to disseminated infections from mycobacteria (including nontuberculous, environmental mycobacteria), salmonella (including nontyphi strains), and Bacillus Calmette-Guerin (BCG) vaccinations). Patients should be evaluated for tuberculosis infection prior to initiating treatment (patients should be monitored closely for signs and symptoms of active tuberculosis during and after treatment). Malignancies were reported among subjects who received this medical product (its safety has not been evaluated in patients with history of malignancy or who have a known malignancy). One case of reversible posterior leukoencephalopathy syndrome (RPLS) was observed (if RPLS is suspected, treatment should be discontinued). Patients being treated should not receive live vaccines (prior to initiating therapy, patients should receive all immunizations appropriate for age). Non-live vaccinations received during treatment may not elicit a sufficient immune response to prevent disease.