Anastrozole's Mechanism of Action
To understand potential interferences, it's essential to know how anastrozole works. This medication belongs to a class of drugs known as aromatase inhibitors. After menopause, the body continues to produce small amounts of estrogen using an enzyme called aromatase. Anastrozole works by blocking this enzyme, thereby reducing the amount of estrogen available to fuel the growth of hormone-receptor-positive breast cancer cells. Because its purpose is to reduce estrogen, any substance that adds estrogen back into the body or counteracts this process can undermine the treatment.
Major Drug-Drug Interactions
Certain medications are highly contraindicated with anastrozole as they directly oppose its mechanism of action. The most critical interactions involve other hormonal therapies and a different type of breast cancer medication.
Estrogen-Containing Products
Taking any form of medication that contains estrogen can make anastrozole much less effective. Since the drug works to lower estrogen levels, adding external estrogen reverses its therapeutic benefit. This includes:
- Hormone Replacement Therapy (HRT): Used to treat menopausal symptoms, HRT is contraindicated for patients on anastrozole.
- Birth Control: Hormonal contraceptives such as pills, patches, and vaginal rings contain estrogen and should not be used.
- Vaginal Estrogen Products: Creams, tablets, or vaginal rings containing estrogen to treat vaginal dryness related to menopause are also contraindicated. Alternatives like non-hormonal lubricants or moisturizers are recommended.
Tamoxifen
Perhaps the most notable interaction is with tamoxifen, another common breast cancer treatment. Patients should not take tamoxifen and anastrozole concurrently unless specifically instructed by a doctor, and concurrent use is generally avoided. In one clinical trial, combining tamoxifen and anastrozole decreased the plasma concentration of anastrozole by 27%, reducing its effectiveness. While both are hormone therapies, they work in different ways and should not be used together.
Herbal Supplements and Phytoestrogens
Many herbal remedies and supplements can interfere with anastrozole, especially those marketed for menopausal symptom relief. This is because they may contain phytoestrogens—plant-derived compounds with weak estrogen-like effects.
Supplements to avoid due to phytoestrogens:
- Soy protein supplements and isoflavones
- Black cohosh
- Red clover
- Dong Quai
- Flaxseed supplements
- Evening primrose oil
- Licorice root
- Ginseng
- DHEA supplements
Other problematic supplements:
- St. John's Wort: Used for depression and anxiety, this supplement can interact with many medications, including anastrozole, and should be avoided.
- Whey Protein: Some sources suggest avoiding this as a concentrated source of phytoestrogens.
Lifestyle and Dietary Considerations
While less direct than drug interactions, certain lifestyle choices and dietary habits can impact treatment effectiveness or exacerbate side effects.
Alcohol Consumption
While a moderate amount of alcohol may not directly stop anastrozole from working, excessive consumption should be avoided. High alcohol intake can increase estrogen levels, working against the medication's purpose. Additionally, alcohol can worsen certain anastrozole side effects like hot flashes, nausea, and dizziness, and may increase the risk of liver problems.
Foods Affecting Side Effects
Though no specific foods are known to directly interfere with anastrozole's efficacy, some can worsen common side effects. Patients experiencing nausea might want to limit or avoid spicy, fried, or greasy foods. Similarly, avoiding caffeine and spicy foods may help manage hot flashes.
Potential Drug Metabolism Interactions
A variety of other medications may have a moderate or minor interaction with anastrozole by affecting its metabolism. The liver enzymes that process anastrozole can be influenced by other drugs, potentially altering the anastrozole levels in the body. It is crucial to inform your doctor and pharmacist of all medications you are taking.
Comparison of Major Interfering Factors
| Interfering Factor | Type of Interference | Impact on Anastrozole | Recommended Action |
|---|---|---|---|
| Hormone Therapy (HRT) | Direct antagonism (adding estrogen) | Counteracts treatment, rendering it ineffective. | Avoid or use alternate drug; inform your doctor. |
| Tamoxifen | Unspecified mechanism, decreases plasma concentration | Makes anastrozole less effective when taken concurrently. | Avoid concurrent use. |
| Phytoestrogen Supplements | Weak estrogen-like effect | Could potentially oppose the anti-estrogen effect. | Avoid supplements; discuss herbs with your doctor. |
| Alcohol | Increases estrogen and exacerbates side effects | May decrease efficacy at high levels; worsens side effects. | Limit or avoid consumption; discuss with your doctor. |
| St. John's Wort | Herbal drug interaction | Could affect metabolism; potential for reduced efficacy. | Avoid use; inform your doctor of all supplements. |
Important Considerations and Conclusion
Maintaining an open and honest dialogue with your healthcare providers is the most important step for safe and effective treatment with anastrozole. Before starting, stopping, or changing the dosage of any medicine, supplement, or herbal product, you should consult with your doctor or pharmacist. They can help you assess the potential risks and ensure your breast cancer treatment remains as effective as possible. A balanced diet and mindful lifestyle choices can also help manage side effects and support overall health during your treatment journey. By carefully managing interactions and adhering to medical advice, you can maximize the benefits of anastrozole and reduce the risk of unwanted complications.
National Institutes of Health (NIH): Breast Cancer Treatment
