Two prostate cancer drugs can make a heart test read falsely high
What happened
The Medicines and Healthcare products Regulatory Agency has warned that two medicines used for prostate cancer can interfere with a common blood test. The warning appears in the MHRA's Safety Roundup for September 2026, published on 29 September. The two medicines are apalutamide and enzalutamide. Both are androgen receptor inhibitors, used to treat prostate cancer. The test affected measures the level of digoxin in the blood. Digoxin is an older heart medicine, used for some heart rhythm problems and for heart failure. The problem is specific. It happens with one method of measuring digoxin, called the chemiluminescent microparticle immunoassay, usually shortened to CMIA. On that test, apalutamide and enzalutamide can make the digoxin reading come out higher than it really is. The MHRA explains why. Both cancer medicines have a chemical shape that is similar to the part of the digoxin molecule the test is designed to latch onto. The test, in effect, mistakes one for the other. There is a detail worth repeating. This can happen whether or not the patient is actually taking digoxin at all.
Why this matters
Digoxin is what doctors call a narrow therapeutic index medicine. That is a technical way of saying the gap between a helpful dose and a harmful one is small. Too little and it does not work. Too much and it can cause sickness, confusion, visual disturbance and dangerous changes in heart rhythm. Because that gap is narrow, doctors measure the level in the blood and adjust the dose to fit. The blood test is not a formality. It drives the decision. So a test that reads too high can lead somewhere unhelpful. A doctor looking at an unexpectedly high number might reduce the dose of a medicine that was working perfectly well, or stop it altogether. For someone whose heart rhythm was being held steady by that medicine, that is not a harmless correction. The other scenario is stranger. Someone who has never taken digoxin could return a digoxin level that looks real. Without knowing about this interference, that result is hard to explain and easy to act on. Prostate cancer is common, and so is heart disease. A lot of men are treated for both. This is not a rare overlap.
What the evidence actually says
A few things are worth being precise about. This is a laboratory interference, not a drug interaction inside the body. The cancer medicines are not making digoxin build up. They are confusing the machine that measures it. It is also specific to one assay method. The MHRA names the CMIA method. Other ways of measuring digoxin are not affected in the same way, which is exactly why the agency's advice is to confirm an odd result using a different method before changing anything. The MHRA's instructions are practical rather than dramatic. Healthcare professionals are told to interpret CMIA digoxin results with caution in patients taking apalutamide or enzalutamide. Raised or unexpected results should be confirmed with an alternative assay that is not affected, before any decision is made about adjusting or stopping digoxin. Clinicians are also asked to consider reviewing results reported in the past. Laboratories are asked to consider adding an advisory comment to serum digoxin reports. The product information for both medicines has been updated to include this risk. The MHRA notes that the issue was also raised in a recent safety communication for enzalutamide. No one is being told to stop either cancer medicine. Nothing here suggests apalutamide or enzalutamide is unsafe. The safety issue is about how a number is read.
Practical advice
If you or someone you care for is taking apalutamide or enzalutamide: - Make sure every clinician who orders your blood tests knows you are on it. That includes your GP, your cardiology team and any hospital you attend in an emergency. - Keep an up-to-date list of all your medicines in your wallet or on your phone. This is the single most useful habit for avoiding mix-ups of exactly this kind. - If you are told your digoxin level is high, it is entirely reasonable to ask whether the result has been confirmed using a different testing method. - Do not stop or change your dose of digoxin, apalutamide or enzalutamide on your own. If you are worried, ring your specialist team or GP. If you have never taken digoxin and a digoxin level is reported for you, mention your cancer medicine and ask for the result to be checked. If you take digoxin and are about to start a new cancer treatment, it is worth raising this with your oncology team so they can flag it to the laboratory.
What to know
The MHRA has warned that apalutamide and enzalutamide, two prostate cancer medicines, can cause falsely high digoxin blood test results when the CMIA method is used. It can happen even in people who are not taking digoxin, because the cancer medicines resemble the part of the digoxin molecule the test recognises. Clinicians should treat raised or unexpected CMIA digoxin results with caution in these patients, confirm them with a different method before changing treatment, and consider reviewing earlier results. Laboratories are asked to add an advisory comment to digoxin reports. Product information for both medicines has been updated. For patients, the action is simple: keep your medicine list current, tell every clinician what you are taking, and ask whether an unexpected digoxin result has been confirmed another way. Sources: Medicines and Healthcare products Regulatory Agency, "MHRA Safety Roundup: September 2026", 29 September 2026, https://www.gov.uk/drug-device-alerts/mhra-safety-roundup-september-2026. NHS, "Digoxin", https://www.nhs.uk/medicines/digoxin/. Cancer Research UK, "Enzalutamide", https://www.cancerresearchuk.org/about-cancer/treatment/drugs/enzalutamide. This article is for general information and does not replace advice from a doctor, pharmacist or other qualified healthcare professional. Never stop or change a prescribed medicine without speaking to your prescriber first.
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