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	The Pharmaceutical Journal Vol 269 No 7227 p812 7 December 2002	This page Reprint Photocopy
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Food-drug interactions

Grapefruit or grapefruit juice?

Clarification, please

Grapefruit or grapefruit juice?

From Dr B. O. Hughes, MRPharmS

In your continuing professional development series (PJ, 19 October, p571) Pamela Mason highlights the interactions between grapefruit juice and drugs such as dihydropyridine, calcium channel blockers and terfenadine (PDF 165K), but your illustration shows half grapefruits with a caption "Grapefruit may interact with many medicines". I understood the interaction to be only with processed grapefruit juice. Could you clarify the matter?

Bryn Hughes
New Malden, Surrey

Dr PAMELA MASON replies: The interaction is possible with any grapefruit juice, whether processed or freshly squeezed. The culprit ingredients appear to be the flavonoids, specifically naringin, concentrations of which may range from 100–800µg/L in any grapefruit juice and have been reported to be 100–500µg/L in commercial grapefruit juice preparations.¹

Reference

1. Fuhr U. Drug interactions with grapefruit juice. Drug Safety 1998;18:251–72.

Clarification, please

From Mr C. E. Richman, MRPharmS

Having read your continuing professional development article (PDF 145K) regarding drug and nutrient interactions (PJ, 26 October, p609), I believe that there were some notable omissions and also reference to one proposed interaction which I have been unable to validate.

I would be interested to know the mechanism or reference for the interaction of verapamil and calcium. There is no indication of interaction in the British National Formulary and one case report of oral calcium supplements in the 1996 edition of 'Drug interactions' by Ivan Stockley (p452). This reference discusses an isolated report of a potential interaction with calcium adipate and calciferol and verapamil, in a patient with diffuse osteoporosis and long standing atrial fibrillation. The commentary suggests that the general importance of this case is unclear.

Notable exceptions in the article's table for calcium are the 4-quinolone group, tetracyclines and thyroxine, all of which are established or well-reported interactions.

I would be grateful to know of any references which further supports the table and would make the general suggestion that such tables be clearly referenced in order to improve the value of such work and to supplement the CPD cycle.

Colin Richman
Stockholm, Sweden

Dr PAMELA MASON replies:

I agree that the importance of the verapamil and calcium interaction is unclear. A general reference to support the article's table is: Utermohlen V. Diet, nutrition and drug interactions. In: Shils ME, Olson JA, Shihe M, Ross CA (editors). Modern nutrition in health and disease. 9th ed. Philadelphia: Lippincott Williams & Wilkins; 1998. pp1619–43.

With regard to the omissions Mr Richman mentions, tetracyclines, quinolones and penicillamine appear in the table in relation to mineral supplements. Calcium is included in these.

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