Enhanced hypoprothrombinemia with warfarin due to azithromycin.

OBJECTIVE: To report a case of probable azithromycin-warfarin drug interaction with enhanced hypoprothrombinemic effect of warfarin. CASE SUMMARY: An 83-year-old African American man stabilized on warfarin therapy (10 mg on Wednesdays, 7.5 mg on other days) developed a prolonged prothrombin time one day after starting azithromycin 500 mg. The elevated prothrombin time normalized 3 days after azithromycin was discontinued. After the initial increase in the international normalized ratio, the absence of any significant confounding factors affecting the anticoagulant effect of warfarin in our patient and the numerous reports of such interactions indicate that an interaction between azithromycin and warfarin may have been responsible for the elevated prothrombin time seen in this patient. An objective causality assessment revealed that the adverse event was probably related to the combination of these drugs. DISCUSSION: Azithromycin, unlike erythromycin and clarithromycin, is not known to inhibit the cytochrome P450 enzyme system and is presumed to be the macrolide of choice in patients already on warfarin. However, previously reported cases of azithromycin-warfarin interactions support the possibility that azithromycin does interact with warfarin, although the exact mechanism is not understood. CONCLUSIONS: Azithromycin may interact with warfarin and enhance its hypoprothrombinemic effects. This effect may be delayed for 4-8 days after a course of azithromycin has been completed. Periodic monitoring of the prothrombin time is recommended when using azithromycin in patients taking warfarin.

Lactic acidemia associated with metformin.

OBJECTIVE: To report 2 cases of lactic acidemia associated with the use of metformin in patients with normal renal function. CASE SUMMARY: An 82-year-old African American man and a 76-year-old white man developed an elevated serum lactic acid concentration a few weeks after initiation of metformin therapy for type 2 diabetes. After the patients discontinued metformin, the serum lactic acid concentration normalized in both cases. An objective causality assessment revealed that the adverse drug event was probably related to the use of metformin. DISCUSSION: Metformin interferes with the production and elimination of lactic acid by a variety of mechanisms that are not well understood. Few systematic data are available on changes in plasma lactic acid concentrations in patients with type 2 diabetes and normal renal function. Clinical significance of a high serum lactic acid concentration needs clarification. CONCLUSIONS: Metformin therapy can be associated with subclinical elevation of lactic acid concentration in the absence of renal insufficiency or other contraindications to using this agent in patients with type 2 diabetes. Periodic monitoring of basic metabolic panels may prevent this potentially serious complication of metformin therapy.