The Editorial Executive Committee welcomes letters, which should be less than 250 words. Before a decision to publish is made, letters which refer to a published article may be sent to the author for a response. Any letter may be sent to an expert for comment. When letters are published, they are usually accompanied in the same issue by their responses or comments. The Committee screens out discourteous, inaccurate or libellous statements. The letters are sub-edited before publication. Authors are required to declare any conflicts of interest. The Committee's decision on publication is final.

Letter to the editor

Editor, – In writing on the dental implications of the management of the post-infarct patient (Aust Prescr 1996;19:13), Dr R. Woods appears to have selectively quoted from his reference1 regarding antibiotics and warfarin. He states 'If antibiotics are employed, dentists should be aware that some antibiotics may inhibit the action of warfarin.' This is true for rifampicin, nafcillin, dicloxacillin1 and griseofulvin.2 Dr Woods has not mentioned that Buckley and Dawson1 also reported potentiation of warfarin by erythromycin, metronidazole, chloramphenicol, quinolones, trimethoprim/sulfamethoxazole, sulfonamides, the imidazole antifungals, some cephalosporins and high intravenous dose penicillin. This latter group includes antibiotics more likely to be prescribed by dentists as the former drugs are not on the Schedule of Pharmaceutical Benefits for dentists.3 Antibiotic potentiation of warfarin has also been referred to in the dental literature.4,5

A recent review concluded there was probable potentiation of warfarin by paracetamol and by dextropropoxyphene,6 and potentiation by aspirin was referred to by Buckley and Dawson.1 These are common drugs in dentistry. Given the number of patients on long-term anticoagulants, Dr Woods may care to elaborate on these important interactions.

Dentists should be mindful of both potentiation and inhibition of warfarin. The former is a potential dental emergency with respect to haemorrhage, the latter a potential medical emergency. I feel the evidence would lead dentists to be more concerned about potentiation of warfarin rather than inhibition and suspect this is by far the more likely problem that dentists may cause or have cause to deal with.

Rod Marshall
Lecturer in Periodontology
Faculty of Dentistry
University of Queensland
Brisbane, Qld

Author's comments

Dr R.G. Woods, the author of the article, comments:

I appreciate the interest expressed by Dr R. Marshall and his comments. Dr Marshall is quite correct regarding the effects of antibiotics on the action of warfarin in the drug regimens of post-infarct patients.

The matter of drug interactions with warfarin is summarised in Goodman and Gilman.7

'The list of drugs and other factors that may affect the action of oral anticoagulants is prodigious and expanding. Any substance or condition is potentially dangerous if it alters (1) the uptake or metabolism of the oral anticoagulant or vitamin K; (2) the synthesis, function, or clearance of any factor or cell involved in hemostasis or fibrinolysis; or (3) the integrity of any epithelial surface.'

Coagulation for those on warfarin is unpredictable. It is usual for the INR to be checked regularly and frequently.

Whether to alter coagulation therapy in anticipation of dental treatment is a decision based on clinical judgment, which should be taken jointly by the medical practitioner managing the case and the dentist. Considerations will include the nature of the proposed treatment, its urgency and the possible consequences of a changed anticoagulant regimen.


  1. Buckley NA, Dawson AH. Drug interactions with warfarin. Med J Aust 1992;157:479-83.
  2. Okino K, Weibert RT. Warfarin-griseofulvin interaction. Drug Intell Clin Pharm 1986;20:291-3.
  3. Commonwealth Department of Health and Family Services. Pharmaceutical benefits for dental use. In: Schedule of pharmaceutical benefits. Canberra:Australian Government Publishing Service, 1996:137-58.
  4. Wood GD, Deeble T. Warfarin: dangers with antibiotics. Dent Update 1993;20:350, 352-3.
  5. Wood GD. Antibiotic prescribing and warfarin enhancement [letter]. Br Dent J 1993;175:241.
  6. Wells PS, Holbrook AM, Crowther NR, Hirsh J. Interactions of warfarin with drugs and food. Ann Intern Med 1994;121:676-83.
  7. Majerus PW, Broze GJ Jr, Miletich JP, Tollefsen DM. Anticoagulant, thrombolytic and antiplatelet drugs. In: Goodman Gilman A, Rall TW, Nies AS, Taylor P, editors. Goodman and Gilman's the pharmacological basis of therapeutics. 8th ed. New York: Pergamon Press, 1990: 1311-31.