The degrees of pain induced by the injection of sodium cephalothin (CET), and its degradation products, deacetylcephalothin (DeCET) and deacetoxycephalothin (DeOCET), were examined by means of writhing response of mice. It was found that these drugs caused slightly painful symptoms, and thought to be likely that the acetic acid liberated during storage was responsible for the pain. CET, dissolved in 0.1 M phosphate buffer at pH 6.2 and 7.0, and stored at 4°C, showed no acid formation and no pain increase, but when stored at 37°C, the solution, in spite of its buffer action, showed remarkable pH decrease, and caused the pain to increase. This was also found in the cases with sodium cephacetrile (CEC) and sodium cephapirin (CEP). In descending order of pH and in ascending order of inducing pain were CEP, CET, and CEC.
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