Heparin rebound associated with anticoagulation and bleeding reported to occur occasionally. May cause hypersensitivity reaction. Hypotension, cardiovascular collapse, noncardiogenic pulmonary edema, pulmonary vasoconstriction, and pulmonary hypertension may occur (risk factors include use of high doses or overdose, repeated doses, previous protamine administration (including protamine-containing drugs), fish allergy, vasectomy, severe left ventricular dysfunction, and abnormal preoperative pulmonary hemodynamics). Too rapid administration can cause severe hypotensive and anaphylactoid-like reactions. May be ineffective in some patients following cardiac surgery.