Sheng Jiang Pi

This is the one point about Sheng Jiang Pi a practitioner must not miss. The Chinese materia medica classes the peel of the fresh rhizome as acrid and cool (xin, liang), entering Spleen and Lung, with the single function of moving water and reducing swelling for incipient oedema with difficult urination. The peeled fresh rhizome, Sheng Jiang, and the dried and blast-fried drugs Gan Jiang and Pao Jiang are all classed as warm or hot and are used to release the exterior, warm the middle or stop bleeding. A drug and its own skin are therefore assigned opposite thermal natures, which is why the classical instruction is to peel the ginger when a warming diaphoretic action is wanted and to keep the skin on when a cooling, water-moving action is wanted. Substituting whole fresh ginger, dried ginger or blast-fried ginger for the peel does not give a weaker version of the same drug; it gives a warming drug in place of a cooling one. The three preparations are also chemically distinct: shogaol content rises and gingerol content falls with drying and heating.

This is a case where the evidence argues against the widely repeated warning. In an open-label three-way crossover study, twelve healthy men took a single 25 mg dose of warfarin alone or after seven days of pretreatment with a recommended dose of a quality-controlled ginger product, with dosing continued for seven days afterwards. INR and platelet aggregation were unaffected by ginger alone, and the apparent clearance of both warfarin enantiomers was essentially unchanged, the mean ratio for S-warfarin being 1.05 (90 percent CI 0.97-1.13) and for R-warfarin 1.02 (0.95-1.10). The study used a rhizome preparation, not the peel, and the peel is used in far smaller quantity than a ginger supplement dose, so the reassurance transfers with room to spare.

In hypertensive patients and normal volunteers, 1 g of ginger potentiated the antiplatelet effect of 10 mg nifedipine against collagen-, ADP- and adrenaline-induced platelet aggregation, in the same direction as aspirin. The authors framed the combination as potentially useful rather than hazardous, but the pharmacodynamic additivity is real and matters in a patient already on dual antiplatelet therapy or facing surgery. Again this is rhizome, not peel, and at a dose larger than a peel decoction supplies.

The whole traditional indication for this drug is promoting urination in early oedema, and it appears in Wu Pi San precisely for that. If that effect is real, adding it to furosemide or a thiazide would be additive, and any drug that changes sodium and water handling can change lithium clearance. There is no pharmacological study of ginger peel diuresis in humans, and no measured natriuretic effect for the peel, so this is reasoning from the traditional indication rather than from data. It is recorded because the consequence, in an elderly oedematous patient on a loop diuretic, would be clinically meaningful if it exists.