Explanation: Fluoroquinolone and macrolide antibiotics both can prolong QTc intervals but there is no risk with tetracyclines.

Expected prolongation:
Moxifloxacin = 16-18 ms (highest of the FQs!)
Azithromycin = 5-9 ms

But why does it matter?

The risk of Torsades de Pointes, a potentially fatal arrhythmia, ⬆️ by 5-7% with every 10 ms ⬆️ in QTc 😰

A normal QTc interval is <450 ms in men and <460 ms in women. Risk vs benefit of QTc prolonging drugs should be considered in patients with a long QTc 🫀

Also consider relative risks of individual drugs when weighing risk vs benefit ⚖️

The amount QTc prolongation isn’t equal across a drug class 💊

Let’s use the FQs QTc prolongation as an example: Ciprofloxacin = 2-5 ms
Levofloxacin = 4-15 ms
Moxifloxacin = 16-18 ms

Keep QTc prolongation in mind and prevent patient harm!