Illustration of how tamoxifen works

A team of researchers from the Mayo Clinic released its findings on which drugs should be avoided by women taking the well known anti-breast cancer medication, Tamoxifen. The findings were reported in the on-line edition of  the Journal of Clinical Oncology. These drugs are strong inhibitors of the active ingredient in Tamoxifen.

Because these drugs interfere with the metabolism of Tamoxifen, women taking this medication should familiarize themselves with the following because not all health care providers will be aware of this.

The drugs are:

• certain popular antidepressants – paroxetine (Paxil), fluoxetine (Prozac) buproprion (Wellbutrin) and duloxetine (Cymbalta)
• antipsychotics – thioridazine (Mellaril), perphenazine, and pimozide (Orap)
• cardiac drugs – quinidine and ticlopidine (Ticlid)
• medications for infectious disease – terfenadine (Seldane which was removed from the US markets in 1998) and quinidine
• miscellaneous medications – cinacalcet (Sensipar which is used to treat hyperparathyroidism)

Not all drugs in the above categories need to be avoided – only the ones that are metabolized (broken down in the body) by the cytochrome P450 system, primarily by the enzyme CYP2D6. There are alternatives within each class of drugs, so the ones with that interfere the least with this system should probably be used.

For medications to work  effectively in the body, they must be broken down into the active component of that drug. The active ingredient in Tamoxifen is endoxifen and it is released by the CYP2D6 enzyme system. Any drug that interferes with this system lowers the blood levels and therefore the protective benefits of this medication.

The list of alternative medications that can be used without interference are:

• antidepressants – venlafaxine (Effexor), desvenlafaxine (Pristiq), reboxetine (Edronax), escitalopram (Lexapro) and mirtazapine (Remeron)
• antipsychotics – thiothixene (Navane). clozapine (Clozaril), risperidone (Risperdal), olanzapine (Zyprexa), ziprasidone (Geodon) and quetiapine (Seroquel)
• cardiac drugs – diltiazem (Cardizem)
• medications for infections – indinavir (Crixivan), saquinavir (Invirase), nelfinavir (Viracept) delavirdine (Rescriptor), nevirapine (Viramune) and efavirenz (Sustiva)
• miscellaneous medication – gabapentin (Neurontin)

In addition, many well known antihistamines are intermediate inhibitors of CYP2D6 and should be used cautiously.

If you know anyone on Tamoxifen, be sure to let them know about these recommendations. For more information, follow this link.

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