PO first pass metabolism requires that
the doses are very high, metabolized in the liver
Liver impairment may
interfere with metabolism and kidney may interfere with excretion
Generally
small to moderate doses relieve constant dull pain moderate to large for
intermittent sharp pain
Give smaller dose if effective
More effective at the onset of pain parentaerally
Chronic on regular schedule
Give before coughing and deepbreathing or dressings
Long term use may be for malignant disease