Drug Interactions between st. john's wort and tamoxifen
This report displays the potential drug interactions for the following 2 drugs:
- st. john's wort
- tamoxifen
Interactions between your drugs
tamoxifen St. John's wort
Applies to: tamoxifen and st. john's wort
Consumer information for this interaction is not currently available.
GENERALLY AVOID: Coadministration with potent inducers of CYP450 3A4 may significantly decrease the plasma concentrations of tamoxifen and its major metabolite, N-desmethyltamoxifen. According to the product labeling, tamoxifen is primarily metabolized by CYP450 3A4 to N-desmethyltamoxifen, an antiestrogenic metabolite with similar biological activity to tamoxifen but an elimination half-life that is estimated to be approximately 10 to 14 days versus 5 to 7 days for tamoxifen. N-desmethyltamoxifen itself is further metabolized by CYP450 2D6 to endoxifen, a major metabolite that is thought to be primarily responsible for tamoxifen's therapeutic effect. When a single 80 mg oral dose of tamoxifen was administered to 10 healthy volunteers following pretreatment with the potent CYP450 3A4 inducer rifampin at a dosage of 600 mg once daily for 5 days, tamoxifen peak plasma concentration (Cmax), systemic exposure (AUC) and elimination half-life decreased by 55%, 86% and 44%, respectively, compared to pretreatment with placebo. In addition, rifampin decreased the AUC and half-life of N-desmethyltamoxifen by 62% and 36%, respectively, and increased the Cmax by approximately 50% compared to placebo. These changes indicate that rifampin enhanced the metabolism of tamoxifen during both the presystemic and elimination phases. In another study, patients with glioma who received high-dose tamoxifen with phenytoin (n=15) demonstrated a 60% lower mean plasma tamoxifen concentration than patients not on concomitant phenytoin (n=10), although the difference did not reach statistical significance due to high interpatient variability and low patient numbers. The interaction has also been described in case reports of two tamoxifen-treated patients who had significantly reduced and subtherapeutic endoxifen levels during or after coadministration with a potent CYP450 inducer. One of them was receiving phenytoin and the other, rifampin. Induction of endoxifen clearance via P-glycoprotein-mediated efflux and/or uridine diphosphate glucuronosyltransferase (UGT)-mediated metabolism may be involved. The extent to which other potent CYP450 3A4 inducers may affect tamoxifen and its active metabolites is unknown.
MANAGEMENT: Until more information is available, concomitant use of tamoxifen with potent CYP450 3A4 inducers should be avoided when possible, particularly if they are to be administered for a prolonged period. Otherwise, therapeutic drug monitoring of tamoxifen and endoxifen is recommended to optimize therapy.
Drug and food interactions
tamoxifen food
Applies to: tamoxifen
Talk to your doctor before using tamoxifen with soy products. There is some evidence that substances present in soy may stimulate breast tumor growth and interfere with the action of tamoxifen, although this has not been proven. Whether soy products are effective for hot flashes is also uncertain. It is important to tell your doctor about all other medications you use, including vitamins and herbs. Do not stop using any medications without first talking to your doctor.
St. John's wort food
Applies to: st. john's wort
While you are taking St. John's wort, you must not eat or drink certain foods and beverages that are high in tyramine. Eating these foods while you are taking St. John's wort can raise your blood pressure to dangerous levels. This may cause life threatening symptoms such as sudden and severe headache, confusion, blurred vision, problems with speech or balance, nausea, vomiting, chest pain, seizure (convulsions), and sudden numbness or weakness (especially on one side of the body). Call your doctor at once if you have any of these symptoms. Foods that are high in tyramine include: air dried meats, aged or fermented meats, sausage or salami, pickled herring, and any spoiled or improperly stored beef, poultry, fish, or liver, red wine, beer from a tap, beer that has not been pasteurize, aged cheeses, including blue, brick, brie, cheddar, parmesan, romano, and swiss, sauerkraut, over the counter supplements or cough and cold medicines that contain tyramine, soy beans, soy sauce, tofu, miso soup, bean curd, fava beans, or yeast extracts (such as Marmite). Caffeine intake should be limited as well. You should avoid or limit the use of alcohol while being treated with St. John's wort. Alcohol can increase the nervous system side effects of St. John's wort such as dizziness, drowsiness, and difficulty concentrating. Some people may also experience impairment in thinking and judgment. Talk to your doctor or pharmacist if you have any questions or concerns.
Therapeutic duplication warnings
No warnings were found for your selected drugs.
Therapeutic duplication warnings are only returned when drugs within the same group exceed the recommended therapeutic duplication maximum.
See also
Report options
Drug Interaction Classification
| Highly clinically significant. Avoid combinations; the risk of the interaction outweighs the benefit. | |
| Moderately clinically significant. Usually avoid combinations; use it only under special circumstances. | |
| Minimally clinically significant. Minimize risk; assess risk and consider an alternative drug, take steps to circumvent the interaction risk and/or institute a monitoring plan. | |
| No interaction information available. |
Further information
Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.
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