Hemolytic potential of methocarbamol.

Methocarbamol, a compound related to mephenesin, has in vitro hemolytic potential. A study was performed to determine whether any hemolysis was detectable after intravenous injection. Methocarbamol and its vehicle (50% polyethylene glycol-300) was compared with vehicle alone and with normal saline controls in high- and low-dose regimens in normal volunteers. Significant increases in plasma hemoglobin were detected 30 min after intravenous injection of methocarbamol or its vehicle alone. Maximum initial plasma hemoglobin levels were approximately 10 mg/dl with vehicle alone, but only 4 mg/dl with the methocarbamol added. Serum haptoglobin levels fell after both high-dose methocarbamol and vehicle during the 3-day period of treatment. Hemolysis, though detectable, did not exceed levels found under physiologic circumstances such as exercise, and represents only a small fraction of the normal daily hemolysis of aged erythrocytes.