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I finally understand my therapy. The app reminds me, answers my questions — and I don't feel alone with it anymore.
Tamoxifen is an anti-oestrogen used after hormone-sensitive breast cancer to lower the risk of recurrence over many years. It only works if it is taken regularly — for five, often ten years, usually without you noticing anything except the side effects. There is a further peculiarity: tamoxifen is only properly activated in the body by the enzyme CYP2D6, and some medicines can slow down exactly that step.
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Tamoxifen needs the liver enzyme CYP2D6 to be converted into its highly active form, endoxifen. Some medicines inhibit this enzyme markedly; taken at the same time, they lower the endoxifen level. The SmPC therefore recommends avoiding strong CYP2D6 inhibitors during tamoxifen treatment wherever possible.¹
An honest assessment: studies do not give a consistent answer as to whether the lower endoxifen level actually leads to more recurrences. Because equivalent alternatives exist for most of these medicines, the practical consequence is nonetheless clear: where a combination can be avoided, it is avoided.²
The most important example is two frequently prescribed antidepressants from the SSRI group: paroxetine and fluoxetine, both very strong CYP2D6 inhibitors. The situation arises easily: after cancer, low mood and anxiety are common, hot flushes add to the strain — and SSRIs are used for both.
Some people, for hereditary reasons, produce hardly any CYP2D6 and naturally have lower endoxifen levels. A genetic test before every treatment would seem the obvious step, but the evidence on its benefit for the course of the disease is contradictory; according to current knowledge, routine testing is not recommended.² The lever you have in your own hands is the medicines you take in addition.
The table summarises the most important combinations; it does not replace an individual review of your medication plan.¹
| Combination | Consequence | What to do |
|---|---|---|
| Paroxetine, fluoxetine, bupropion, terbinafine (strong CYP2D6 inhibitors) | Less endoxifen, possibly a weaker effect | Avoid where possible; discuss an alternative with the practice, do not stop abruptly |
| Coumarin anticoagulants such as phenprocoumon | Markedly increased anticoagulation, risk of bleeding | Monitor the INR closely, especially at the start and with any changes |
| Aromatase inhibitors (at the same time) | No added benefit, the effect may be weakened | One after the other rather than at the same time, as planned by the team |
| Oestrogen-containing preparations (hormone replacement, hormonal contraception) | Opposing effect, unsuitable with a hormone-sensitive tumour | Do not use; discuss hormone-free alternatives |
| Strong enzyme inducers such as rifampicin, also St John's wort | Lower tamoxifen levels possible | Do not combine on your own; see herbal medicines |
| High-dose isoflavones, red clover, soya concentrates | Oestrogen-like plant substances, safety unclear | Not recommended as supplements; a normal diet containing soya is considered safe |
| Alcohol | No direct interaction known, but an independent risk factor for breast cancer | Drink as little as possible |
The real risk seldom lies in a single prescription, but in the fact that over five to ten years many practices prescribe something: the GP the antidepressant, the dermatologist the antifungal, the cardiologist the anticoagulant. An up-to-date medication plan and an interaction check before you start anything new are therefore particularly valuable with tamoxifen — supplements included.
Hot flushes are the most common reason for struggling with tamoxifen — and the area where well-meant self-help can undermine the treatment: hormones are ruled out, so are certain antidepressants, and many herbal remedies have oestrogen-like effects. Even so, quite a lot remains.²,³
Tamoxifen treatment usually runs within structured breast cancer follow-up care, at shorter intervals in the first few years and at longer intervals later. The tolerability of the treatment is part of these appointments too.²
| Area | What is checked | Do not wait for the next appointment if you have |
|---|---|---|
| Womb | Gynaecological examination; routine ultrasound without symptoms is generally not recommended, because tamoxifen often makes the lining look thickened | Bleeding after the menopause, new bleeding between periods, unusual discharge |
| Veins | Thrombosis risk, prophylaxis before operations | Swelling of one leg, breathlessness, chest pain |
| Eyes | Eye examination if your vision changes | Blurred vision, new sensitivity to glare |
| Liver, blood count | Laboratory values as directed by the practice | Yellowing of the skin, dark urine, persistent upper abdominal discomfort |
Note down what you observe between appointments: when did bleeding occur, how often did hot flushes come, how long have you had calf cramps? A record over time makes it easier to judge whether something has changed than a memory does.
Tamoxifen must not be used during pregnancy or breastfeeding. Before the menopause, reliable non-hormonal contraception is needed during treatment and, according to the SmPC, for about two months after stopping — for example a copper coil or barrier methods.¹,⁵ If you become pregnant nonetheless, inform your treatment team immediately; embryotox also offers individual risk counselling.
Many young women ask whether their wish for children has to wait five to ten years. An international study (POSITIVE) suggests that a planned interruption of anti-hormonal therapy after at least a year and a half, in order to become pregnant, was not associated with more recurrences, at least in the short term. Long-term data are still awaited. Such a break is always planned together with the treatment team and includes resuming treatment afterwards.
Tamoxifen is also the standard treatment for men with hormone receptor-positive breast cancer. Besides hot flushes, men frequently report reduced sex drive and erectile problems — topics that often go unmentioned and are a major reason for stopping. It is worth raising them openly.
Because of the thrombosis risk, tamoxifen belongs in the conversation with the anaesthetist before any operation. According to the SmPC, treatment is interrupted only if the risk of thrombosis clearly outweighs the benefit of taking it without a break; consistent thrombosis prophylaxis is usually more important.¹ The same applies by analogy to longer periods of immobility and to long-haul flights. More in the guide medications before surgery.
Because that is exactly the state tamoxifen is meant to preserve. The treatment is aimed at individual tumour cells that may have remained in the body after the initial treatment and that no examination can detect. Nobody knows whether they exist. Tamoxifen lowers the likelihood that they turn into a recurrence years later. So feeling healthy is not an argument against the treatment but its goal. Ask your treatment team roughly how big the benefit is in your situation — then you decide on an informed basis rather than on gut feeling.
Yes — mention explicitly that you take tamoxifen. Paroxetine and fluoxetine inhibit the enzyme that activates tamoxifen and should be avoided where possible; citalopram, escitalopram or venlafaxine affect it much less. Your doctor makes the choice, but can only make it correctly if they know about the tamoxifen treatment. Low mood after cancer is common and treatable — more on this under depression.
No, do not take several tablets at once; simply carry on with the next regular dose. Thanks to the long half-life, individual gaps have only a small effect on the level. What matters more is why the gap happened — a holiday, an empty pack, a day without your routine? That is exactly where to start, so that three days do not turn into three weeks. Mention the gap at your follow-up appointment; that is normal and helps the team advise you realistically.
The interaction check shows whether a new medicine could slow down the activation of tamoxifen.
