HELEN NEILL, staff editor at the British National Formulary and the British National Formulary for Children, responds:

Mr Potter is right to be concerned that glucosamine might affect the anticoagulant effect of warfarin. The Medicines and Healthcare products Regulatory Agency raised this matter in the May issue of Current Problems in Pharmacovigilance. The BNF will mention the interaction between glucosamine and warfarin in the new edition due out next month.

An information resource can only be a snapshot of knowledge at a given moment and from a particular standpoint. Health professionals recognise that this knowledge needs to be supplemented by an awareness of recent clinical developments. It is easy to be beguiled by online and computer resources into thinking that they deliver up-to-the-minute information. In reality, this cannot be so — systems suppliers themselves need to collect, evaluate, safely embed and then deliver new information and this takes time.

In the case of an interaction between warfarin and glucosamine, there is yet another complication. While warfarin, a conventional medicine, falls firmly within the scope of information resources such as the BNF, glucosamine does not. Drugs can interact in unwanted ways with a wide array of endogenous and exogenous substances, including hormones and foods (the effect of warfarin can be modified by vegetables and salads, alcohol and possibly by substances such as cranberry juice). A drug information resource on conventional medicines cannot easily give comprehensive guidance on all these possibilities. Physicians and pharmacists need to call upon their professional knowledge and training in order to deal with patients’ use of prescription and non-prescription medicines and to determine what action to take, eg, monitor anticoagulant activity more closely.