Plasma concentration, mammary excretion and side-effects of phenylbutazone after repeated oral administration in healthy cows.

Seven clinically healthy dairy cows were each given 2.5 g phenylbutazone (approximately 5 mg/kg body weight) by oral administration twice daily for 8 days. The concentrations of phenylbutazone in plasma and milk and several blood parameters were studied. The minimum plasma concentration during steady state was 100.4 +/- 7.3 micrograms/ml. During the same period the milk concentration never exceeded 1% of the plasma concentration. The elimination half-life in plasma was 38.6 +/- 3.7 h. Five days after administration had been discontinued, the milk concentration was 0.05 +/- 0.01 microgram/ml. All seven cows were clinically healthy throughout the experiment. The most pronounced side-effect of the blood parameters studied was a decreased concentration of leucocytes to about two-thirds of the control value. This might have a pronounced influence on the effectiveness of the immune system. There was also a significant decrease in total bilirubin indicating a decrease in the breakdown of erythrocytes.