Dinesh Mehta (Editor of the BNF) replies:

Two important issues arise from your correspondent’s letter.

First, is there a clinically significant interaction between lacidipine and terfenadine?

A close reading of the SPC for Motens suggests a theoretical possibility that lacidipine might prolong the QT interval on the basis that caution is advised for other (unnamed) dihydropyridine calcium channel blockers. The SPC extrapolates this to say that lacidipine should be “used with caution” in those receiving (unnamed) antihistamines. However, the SPC section on undesirable effects does not say that lacidipine prolongs the QT interval. Furthermore, the section on interactions does not mention an interaction with drugs that prolong the QT interval. On the contrary, the SPC reassuringly points to the lack of interactions with diuretics (which have been reported to prolong the QT interval by affecting electrolyte balance).

Like the BNF, Ivan Stockley’s Drug Interactions (fifth edition) also does not mention the possibility of an interaction between lacidipine and terfenadine.

Second, pharmacists and prescribers should choose a drug on the basis of all its properties. A potential for drug interaction is not the sole determinant of suitability for treatment; cautions, contra-indications and side-effects of the drug also need to be considered.

The BNF clearly states that terfenadine should be avoided in known or suspected prolonged QT interval. A careful prescriber would avoid the drug at the merest hint of cardiac conduction disorders including QT interval prolongation.

It is incumbent on those who provide information on medicines to interpret SPCs and other sources of information wisely.

Information needs to be clinically relevant and it should be presented in a balanced way. Information providers need to understand that spurious information may divert a health care professional to using a less safe alternative.