The combination of Bactrim (sulfamethoxazole-trimethoprim) and Diflucan (fluconazole) may cause an increased risk of QT prolongation. QT prolongation may cause rare, but severe heart rhythms known as ventricular tachycardia and torsade de pointes. Many drugs may cause a prolonged QT interval and each additional drug that has this effect increases the overall risk of developing one of these arrhythmias.  

Depending on other risk factors, such as previous history of heart disease, QT prolonging drugs may be prescribed together as long as the patient and prescriber are aware of the potential risk. Other risk factors include electrolyte imbalance and additional proarrhythmic conditions. Additionally, a baseline electrocardiogram may be helpful to understand the risk for each patient.

Since Bactrim and Diflucan are generally used for a short-term course, the risk of taking these together would be less than if they were being taken chronically. With a one-time dose of Diflucan, the risk of causing a severe arrhythmia would be low. As long as the prescriber is aware and other risk factors are considered, this combination may be completely appropriate. The alternative would be to delay treatment of the bacterial infection, which may prolong symptoms and worsen the infection.

The half-life for Diflucan usually ranges between 20 and 50 hours (approximately 30 hours in most patients). The drug would be completely out of the system by one week, but the risk would be significantly reduced even after 1 day (almost 50% of the drug would be out of the system).

Bactrim contains two active ingredients, sulfamethoxazole and trimethoprim. The half-life for sulfamethoxazole is 6-12 hours and the half-life for trimethoprim is 8-10 hours. The sulfamethoxazole component contains a sulfa group and should not be taken in patients with a sulfur allergy.