Sir, we are writing to draw attention to some interesting research that questions the validity of the disulfiram-like reaction between metronidazole and alcohol. This reaction is the reason the British National Formulary1 advises to avoid alcohol during and for 48 hours after taking metronidazole. Giving this advice is standard practice amongst most clinicians.

Disulfiram is a drug used to discourage alcohol consumption. Its interaction with alcohol leads to acetaldehyde accumulation causing symptoms such as skin redness, palpitations, nausea, vomiting, headache and in severe cases circulatory collapse.2 The disulfiram-like reaction of metronidazole and alcohol is said to be similar, and was traditionally explained by the same mechanism, although this now seems to be incorrect.2,3,4 Its frequency is unclear as figures vary between 0 and 100%.5

Its validity has been repeatedly questioned in the modern literature. Serious reactions including at least one death have been attributed to it,3,5 although at least some of these have been disputed.3 A number of clinical studies and reviews have found evidence of the existence of this interaction to be absent or weak.2,3,4,6

Although we do not seek to promote alcohol intake, the advice to abstain completely will restrict patient lifestyle for that period. There are situations such as alcohol dependent patients where this could be especially problematic, so settling this is important.

Overall the evidence for this reaction appears to be weak at best. It appears likely that the concern attached to it is overstated. The purported reaction could actually be an alcohol-independent side effect of metronidazole, an effect of alcohol, or disease - possibilities not adequately eliminated by the studies.2 Furthermore, the term 'disulfiram-like' is a misnomer, at least in a biochemical sense, as it seems that any such reaction does not occur through the same mechanism as disulfiram. Conversely, no definite evidence is presented that this reaction does not occur - perhaps it occurs only in a small subgroup. The aim of this letter is not to suggest we, as clinicians, stop advising patients to avoid alcohol whilst on metronidazole. Rather all clinicians should be alert to its weak evidence base and be ready to question and reject long-held beliefs and mantras such as this should new evidence emerge.