Dexamethasone concentrations in bovine blood plasma and milk after intravenous injection.

Passage of dexamethasone from plasma to milk in five lactating dairy cows after an intravenous injection was evaluated by a high performance liquid chromatographic technique for measurement of concentrations in blood plasma and milk. The dexamethasone ester was 21-isonicotinate as a solution and was administered at .1 mg/kg bodyweight. The drug was detected in milk postinjection from 15 min to 8 h with a peak concentration of 20.6 ng/ml at the second sample time (30 min). Half-times of dexamethasone in plasma and milk were 4.5 and 3.0 h. The ratio of mean concentration for milk/plasma was .39. It is anticipated that no residues of dexamethasone would be detected in milk if normal dairy practices are followed.