- Action to be taken: Caution
- Rationale: In vitro metabolism of prazosin in animals or humans has not been reported. Early metabolism studies with 14C-labeled prazosin in rats and dogs revealed that it undergoes extensive hepatic metabolism with biliary excretion as the major route of elimination. Regardless of its metabolism it does not seem to interact with other drugs via cytochrome P450 pathways.
- Cytochrome P450 enzyme substrate: CYP2D6 and CYP3A4
- Action to be taken: Caution with close monitoring for side effects
- Rationale: In vitro studies have shown that marijuana has a minimal impact on CYP3A4 and CYP2D6. Nonetheless, given the lack of inhibition data at several CYP-450 isoforms,clinically significant inhibitory effects cannot be ruled out entirely. Caution is advised when administering tamsulosin with CYP3A4 inhibitors, such as azole antifungals. Tamsulosin is extensively metabolized by CYP3A4.Clinical evaluation of concomitant use of tamsulosin with a strong CYP3A4 inhibitor resulted in significant increases in tamsulosin exposure; interactions with moderate CYP3A4 inhibitors have not been evaluated. If concomitant use is necessary, patients should be monitored closely for increased side effects.
- Flomax (tamsulosin) package insert. Ridgefield, CT: Boehringer Ingelheim Pharmaceuticals, Inc.; 2014 Oct.
- Hansten P, Horn J. The Top 100 Drug Interactions: A Guide to Patient Management. includes table of CYP450 and drug transporter substrates and modifiers (appendices). H & H Publications, LLP 2014 edition.
- Watanabe K, Yamaori S, Funahashi T, et al. Cytochrome P450 enzymes involved in the metabolism of tetrahydrocannabinols and cannabinol by human hepatic microsomes. Life Sci. 2007;80:1415-1419.
- Stout SM, Cimino NM. Exogenous cannabinoids as substrates, inhibitors, and inducers of human drug metabolizing enzymes: a systematic review. Drug Metab Rev. 2014;46:86-95.
- Flockhart DA, Tanus-Santos JE. Implications of Cytochrome P450 Interactions When Prescribing Medication for Hypertension. Arch Intern Med. 2002;162(4):405–412. doi:10.1001/archinte.162.4.405