The most commonly reported adverse reactions in clinical trials in previously treated patients (PTPs) ? 12 years of age (? 5%) were headache, cough, nausea, and fever ^{FDA label}.

Hypersensitivity reactions, which includes severe allergic reactions, have occurred. Monitor patients for hypersensitivity symptoms. Should hypersensitivity symptoms occur, stop treatment with this agent and administer appropriate supportive treatment. Hypersensitivity reactions may also be related to antibodies targeted against polyethylene glycol (PEG) ^{FDA label}.

Development of Factor VIII neutralizing antibodies can also occur. If expected plasma Factor VIII activity levels are not reached, or if bleeding is not controlled as expected with the administered dose, perform an assay that quantifies Factor VIII inhibitor concentration ^{FDA label}.

Immune response to PEG, manifested as symptoms of acute hypersensitivity and/or loss of drug effect, has been observed mainly in subjects less than 6 years of age. Evaluate patients experiencing symptoms of hypersensitivity reactions in the absence of detectable Factor VIII inhibitors for possible bleeding or impaired recovery ^{FDA label}.