Some readers have asked why the June 2009 issue of Treatment Guidelines (Antimicrobial Prophylaxis for Surgery) did not recommend use of ertapenem (Invanz) for prevention of infection after elective colorectal surgery. Ertapenem is a broad-spectrum carbapenem that has been approved for such use by the FDA. Medical Letter consultants do not recommend use of broad-spectrum drugs such as ertapenem, third-generation cephalosporins such as cefotaxime (Claforan), ceftriaxone (Rocephin), cefoperazone (Cefobid), ceftazidime (Fortaz, and others) or ceftizoxime (Cefizox), or fourth-generation cephalosporins such as cefepime (Maxipime) for routine surgical prophylaxis because they are expensive, some are less active than first- or second-generation cephalosporins against staphylococci, and their spectrum of activity includes organisms rarely encountered in elective surgery. These drugs should be reserved for treatment of serious infections, particularly those likely to be caused by organisms resistant to other antimicrobials.

A reader expressed disappointment that our recent listing of “Some Warfarin Drug Interactions”1 did not include acetaminophen. Perhaps it should have. Acetaminophen can increase the anticoagulant effect of warfarin, particularly with continued use, but it does so inconsistently. The mechanism of this interaction has not been established, but may be related to an acetaminophen metabolite inhibiting vitamin K-epoxide reductase, the target for warfarin’s anticoagulant effect.2

Patient susceptibility varies, possibly on a genetic basis; occasional use of acetaminophen generally has little or no effect on the international normalized ratio (INR) in patients on chronic warfarin therapy, but in some, even a few grams of the drug may cause a dramatic increase in INR. One study in healthy subjects found no effect of acetaminophen 4 g per day for 2 weeks, while another study in patients with the same acetaminophen dose for the same period of time found a moderate increase in INR.3,4 It might be prudent to monitor INR in patients on chronic warfarin therapy more closely than usual when they take more than 2 g per day of acetaminophen for more than a few days.

1. Pharmacogenetic-based dosing of warfarin. Med Lett Drugs Ther 2008; 50:39.
2. HH Thijssen et al. Paracetamol (acetaminophen) warfarin interaction: NAPQI, the toxic metabolite of paracetamol, is an inhibitor of enzymes in the vitamin K cycle. Thromb Haemost 2004; 92:797.
3. D Kwan et al. The effects of acetaminophen on pharmacokinetics and pharmacodynamics of warfarin. J Clin Pharmacol 1999; 39:68.
4. I Mahe et al. Paracetamol: A haemorrhagic risk factor in patients on warfarin. Br J Clin Pharmacol 2005; 59:371.

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(Vol. 49, p. 66, August 13, 2007) A reader has pointed out that Saccharomyces boulardii is not a separate species, but a strain of Saccharomyces cerevisiae. S. cerevisiae (including S. boulardii) has been reported to cause systemic infection after oral ingestion in both immunocompromised patients and healthy hosts (MJ McCullough et al. J Clin Microbiol 1998; 36:2613).

An article (Med Lett Drugs Ther 2007; 49:41) on sunscreens in the May 21 issue of The Medical Letter included a statement that if sunscreens and the insect repellent DEET are used together, DEET should be applied first to avoid decreasing the effectiveness of the sunscreen. However, a recent in vitro study in human skin found that sunscreen increases absorption of DEET, especially when DEET is applied first (T Wang and X Gu. J Pharm Pharm Sci 2007; 10:17). Whether this practice could cause clinical toxicity is unclear. Some travel experts recommend applying sunscreen first.

Since the publication of our article on influenza vaccine (October 9, 2006), a fourth inactivated vaccine (FluLaval - GlaxoSmithKline) has been approved by the FDA for use in people ≥18 years old. It is sold in 5-mL multi-dose vials. Each 0.5-mL intramuscular dose contains 25 mcg of mercury.