Bioavailability of phenylbutazone preparations in the horse

Plasma phenylbutazone concentrations were determined for up to 12 h in 6 horses following intravenous and oral phenylbutazone administration. To evaluate the bioavailability of different oral preparations, phenylbutazone was administered in a paste as well as the traditional powder form. The effect of the state of stomach contents on the absorption of phenylbutazone was investigated by administering the paste before and after feeding; the powder was given in a small bran mash and a full feed of lucerne chaff, wheaten chaff and bran. Despite great variability among individual horses both the paste form of phenylbutazone administered before a meal and the powder given in a small bran mash appeared to be almost completely absorbed, However, phenylbutazone paste administered after feeding or phenylbutazone powder given in a full feed resulted in lower peak plasma phenylbutazone levels than when administered on an empty or relatively empty stomach.