In this episode, we “deep dive” into diltiazem, describing its most important drug facts, pharmacology and medicinal chemistry, pharmaceutics, AB compatibility, and important medication safety issues. Key Concepts Diltiazem is a non-dihydropyridine calcium channel blocker (CCB). This type of CCB reduces both heart rate and blood pressure whereas dihydropyridine CCBs only reduce blood pressure. Diltiazem has numerous dosage forms (IV, immediate release tablets, and extended-release products). Extended-release products are always dosed once or twice daily. Historically there were a significant number of extended-release capsules with a variety of brand names and AB-compatibility. Today, only a few branded products still exist in the US market (Cardizem CD, Cartia XT, Cardizem LA, Tiazac, Taztia XT). The FDA Orange Book describes “AB” compatibility, which outlines whether one formulation is therapeutically equivalent to another formulation. Depending on state law, pharmacists can use AB compatibility codes to automatically substitute formulations without notifying the prescriber. The numerous dosage forms of diltiazem is a medication safety issue. Remember that immediate release diltiazem is always dosed TID/QID (3-4 times per day) whereas extended-release formulations are always dosed once daily. A twice-daily extended-release product was previously on the market but has since been discontinued.