Looking after our health has become a priority, and it is common nowadays to have supplements such as vitamin C, magnesium, melatonin or herbal extracts at home. It is easy to assume that, as they are ‘natural’ and over-the-counter products, they are completely harmless.
However, at HC Marbella International Hospital, we would like to share with you an important finding backed by recent scientific evidence: supplements can be excellent allies, but without supervision, they can put your medical treatment at risk.
Imagine Carmen, a fictional patient who visits our clinic. She takes daily blood pressure medication and, lately, has been feeling a bit down, so she decides to buy capsules of St John’s Wort (a popular herbal remedy for mood). As it is ‘natural’, she doesn’t mention it to her doctor. Weeks later, her blood pressure medication stops being effective. What has happened?
Our liver acts like a customs checkpoint that processes everything we swallow. Certain plants and supplements force this ‘customs checkpoint’ to work faster or slower. In Carmen’s case, the herb caused her body to eliminate the medication before it could take effect.
If you take daily medication for your heart, thyroid or blood sugar, or are undergoing cancer treatment, adding vitamins on your own is like inviting too many people into a small kitchen: they end up getting in each other’s way.
Medical studies warn of combinations that occur every day:
- If you take this supplement:St John’s Wort
And this medication: Anticoagulants or immunosuppressants
What happens in your body: The medication loses its effectiveness.
- If you take this supplement:Ginkgo biloba or garlic
And this medication: Sintrom or aspirin (Adiro)
What happens in your body: The risk of bleeding increases.
- If you take this supplement: Calcium (in high doses)
And this medication: Thyroid tablets
What happens in your body: Your gut does not absorb the medication.
It’s easy to think that a megadose of vitamins will make us stronger, but excess can have consequences:
- Vitamin D: Living on the sunny Costa del Sol, many of us already have good levels. Taking massive doses without a prior blood test can cause calcium to build up in the blood, leading to kidney damage and arrhythmias.
- Antioxidants (Vitamins C and E): In excess, vitamin E can affect blood clotting. Furthermore, in patients undergoing chemotherapy, high doses of antioxidants may protect tumour cells rather than destroy them.
Your safety comes first. To enjoy the benefits of supplements without taking any unnecessary risks, we suggest following these three very simple steps:
- The ‘Bag Technique’: On your next visit to HC Marbella, put all the supplements, vitamins and herbal teas you are taking into a bag and bring them to your consultation. Your doctors and nurses need to see exactly what you are taking to tailor your treatments.
- Do not self-diagnose: If you feel tired or think your immune system needs a boost, do not go straight out to buy vitamins. Ask us for a blood test. We will tell you exactly what you need and in what dosage.
- Ask the experts, not the internet: It has been shown that Artificial Intelligence and internet searches fail more than half the time when trying to predict whether a vitamin will interact with a medicine. Always put your trust in your healthcare team or your pharmacist.
Conclusion
Dietary and vitamin supplements are not inherently dangerous. But neither are they inherently safe. They are bioactive substances with real pharmacological effects that can interfere with established medical treatments, cause direct toxicity at inappropriate dosages, and create a false sense of security that discourages people from seeking appropriate medical care.
Recent evidence published in high-impact scientific journals—from the Journal of the American Geriatrics Society to Nutrients, Current Medicinal Chemistry or Clinical and Translational Science—all point in one unequivocal direction: we need to approach supplementation with the same clinical rigour as conventional pharmacology. The first step is to break the silence during the consultation.
No question is too small. If you are taking any natural products, please mention this at your next appointment. We are here to ensure that everything you take contributes to your health rather than creating problems.
July 3, 2026
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