Dutasteride is metabolized in the liver by the CYP3A4 and CYP3A5 isoenzymes. No clinical drug-interaction study has evaluated the effect of CYP3A inhibitors on dutasteride, but the FDA label states that, on in vitro data, blood concentrations of dutasteride may increase in the presence of CYP3A4/5 inhibitors such as ritonavir, ketoconazole, verapamil, diltiazem, cimetidine, troleandomycin and ciprofloxacin. The label advises caution when prescribing dutasteride to patients on potent, chronic CYP3A4 inhibitors, and recommends no dose adjustment for verapamil or diltiazem, whose effect it does not consider clinically significant. Dutasteride is FDA-approved in the United States for bph/">benign prostatic hyperplasia only; use for hair loss is off-label, and these interactions apply either way.
Drug Interactions
CYP3A4 Inhibitors
Because dutasteride is metabolized by CYP3A4 and CYP3A5, drugs that inhibit those enzymes may increase dutasteride levels. The label does not grade these drugs as strong, moderate or weak. What it does say is this:
- Named in the label: ritonavir, ketoconazole, verapamil, diltiazem, cimetidine, troleandomycin and ciprofloxacin. Dutasteride blood concentrations may increase in the presence of these CYP3A4/5 inhibitors, based on in vitro data
- Measured in a population pharmacokinetics analysis: dutasteride clearance fell 37% with verapamil (n = 6) and 44% with diltiazem (n = 5). Amlodipine, a calcium channel antagonist that is not a CYP3A4 inhibitor, produced no decrease (+7%, n = 4)
- Not studied: no clinical drug-interaction study has been performed to evaluate the impact of CYP3A enzyme inhibitors on dutasteride pharmacokinetics
- Other CYP3A inhibitors you may be taking: FDA’s clinical drug-interaction table lists clarithromycin and itraconazole as strong index inhibitors of CYP3A, and erythromycin, fluconazole and verapamil as moderate index inhibitors. The dutasteride label does not name clarithromycin, itraconazole, erythromycin or fluconazole, so no dutasteride-specific figure exists for them. They are listed here because they act on the same enzyme that clears dutasteride, not because an interaction has been measured
- Grapefruit juice: MedlinePlus advises talking to your doctor about eating grapefruit or drinking grapefruit juice while taking dutasteride. Neither the label nor MedlinePlus quantifies an effect
The label recommends no dose adjustment, and directs prescribers to use caution when dutasteride is given to patients taking potent, chronic CYP3A4 inhibitors. If you take one of these drugs long term, raise it with your prescriber.
Tamsulosin
Dutasteride is commonly co-prescribed with tamsulosin for BPH (marketed as Duodart or Combodart, and in the United States as Jalyn). The label reports that giving dutasteride with tamsulosin or terazosin has no effect on the steady-state pharmacokinetics of either alpha blocker, and that the reverse direction, the effect of tamsulosin or terazosin on dutasteride pharmacokinetics, has not been evaluated. Learn about the Duodart combination.
Other 5-Alpha Reductase Inhibitors
There is no FDA-approved combination of dutasteride and finasteride, and no trial data in either product’s label supports taking them together. Both act on 5-alpha reductase, and dutasteride inhibits both type 1 and type 2 isoenzymes while finasteride is selective for type 2. Neither label describes any added benefit from combining them. Read more.
Alcohol
Alcohol does not appear in the drug-interactions section of the dutasteride label. Dutasteride is cleared by hepatic CYP3A4 and CYP3A5, and the label notes it is not metabolised by CYP2E1, one of the enzymes involved in alcohol metabolism. Heavy chronic drinking can affect liver function generally. Full alcohol interaction guide.
Testosterone and Hormones
Dutasteride raises serum testosterone modestly, because it blocks the conversion of testosterone to DHT. In men with BPH treated for 4 years, the label records a median increase in serum testosterone of 19% at 1 and 2 years, 26% at 3 years and 22% at 4 years, with mean and median levels remaining within the physiologic range. This may be relevant for patients on testosterone replacement therapy (TRT). Learn more about dutasteride and testosterone.
Lab Test Interactions
Dutasteride reduces serum PSA by approximately 50% within 3 to 6 months of treatment. To interpret an isolated PSA value in a man treated for 3 months or more, the label directs that the value be doubled for comparison with normal ranges in untreated men. Dutasteride also reduces serum DHT, with a median decrease of 94% at 1 year, and raises serum testosterone modestly. In healthy volunteers treated for 52 weeks, the label reports no clinically significant change compared with placebo in sex hormone-binding globulin, estradiol, luteinizing hormone, follicle-stimulating hormone, free T4 or dehydroepiandrosterone.
What is dutasteride? · Side effects · Dosage
Sources
- FDA. Avodart (dutasteride) prescribing information, Drug Interactions and Clinical Pharmacology. accessdata.fda.gov
- MedlinePlus. Dutasteride. medlineplus.gov
- FDA. Drug Development and Drug Interactions Table: Substrates, Inhibitors and Inducers. fda.gov