A dentist in my practice prescribed clindamycin to a female patient who required premedication for the prevention of bacterial endocarditis. He prescribed her 300 mg instead of the standard 600 mg because the patient was small in stature. When I asked whether this would provide sufficient coverage, the dentist suggested that a prescription for metronidazole in addition to the clindamycin would ensure the patient was fully protected. I have never heard of dually prescribing antibiotics. Can you shed some light on this practice?
In general, antibiotics are not dually prescribed for several reasons: the practice may compromise the efficacy of one or both antibiotics; it may create negative side effects that impact patient safety and compliance; it could contribute to the promotion of microbial resistance.
There are some circumstances that require dual prescribing. For example, if an orofacial infection is not responding after 3 days of antibiotic use (assuming good patient compliance), the causative bug is assumed to be either resistant to the initial antibiotic or outside of the spectrum of kill. Remember, the drug of choice for orofacial infections is penicillin, a narrow spectrum antibiotic that is effective against Gram-positive Streptococcus and Staphylococcus. Since most orofacial infections are caused by resident organisms of the mouth, the penicillins are typically effective. Broad spectrum antibiotics (eg, metronidazole) are not introduced arbitrarily—the situation must warrant the need to do so. Antibiotic premedication is not one of these situations, as risk is associated with resident organisms of the mouth. For an orofacial infection that is not responding to the narrow spectrum antibiotic, another antibiotic that can overcome resistant organisms (eg, Augmentin) should be tried. Or, a second antibiotic could be added that will broaden the spectrum (eg, metronidazole). The dual prescription of antibiotics is most commonly used in the treatment of aggressive periodontitis. In this case, metronidazole—a broad spectrum antibacterial agent especially effective against susceptible periodontal pathogens—is added.
Metronidazole is not recognized by the American Heart Association (AHA) as an agent of choice for the prevention of infective endocarditis. Adding it to an insufficient dosing regimen of clindamycin will not “make up the difference” because these drugs target different classes of microorganisms.
Clindamycin is prescribed to premedicate patients who are allergic to penicillin. The premedication dose must be adequate for efficacy. While the AHA recommends calculating the dose of the antibiotic for pediatric patients based on mg/kg of body weight, adult dosages are based on the typical 150-pound adult. If the patient is small in stature and more closely resembles a pediatric patient (say around 100 pounds), then the dose should be recalculated—not simply cut in half. The vast majority of people who receive a single dose of clindamycin for premedication tolerate it well without complications. As such, it is not typically necessary to adjust the dose for adults, even when the person is small. When determining the dose for pediatric patients, the calculated dose must not exceed the adult dose. This is becoming more challenging as many children are significantly overweight.
In conclusion, the best plan of action is to follow the posted recommendations for antibiotic premedication, and to collaborate with the patient’s physician and pharmacist if there are any concerns that the posted adult dosage may be too high.