Opinion: I used to think supplements were all much the same, until we started reviewing them

If you're looking for a daily supplement, you might be tempted to go to your local pharmacy and pick out whatever's cheapest, but our testing reveals that's not always the best idea.

The contents of supplements differ more than you might think, and you could end up paying for a less effective tablet - but equally paying a premium doesn't always pay off either.

This column appears in the November 2025 issue of Which? magazine. Join Which? to get our monthly magazine, or if you're already a Which? member, add the magazine to your membership in your online account or by calling us on 029 2267 0000.

Shefalee Loth, principal researcher and nutritionist

For the past yearm I’ve been working with our team of research experts, and consultant dietitian Sophie Medlin, to develop our first ever reviews of vitamin and mineral supplements. This forms part of our mission to increase our coverage of the health and wellbeing products we know you're buying.

I used to think they were all much of a muchness. If someone asked which brand to take, my advice was to go for the cheapest that has the right dose and in your preferred format: tablet, capsule or spray.

My assumptions weren’t completely wrong. The dose is important and, if you don’t like swallowing pills, a spray can be a good alternative. 

Plus, the mantra ingrained in my soul after years of working at Which? holds true: price doesn’t always reflect quality. Some pricey products have proved disappointing, and some budget ones have outclassed far more expensive rivals.

But there are real differences between products, and it's time-consuming and difficult to determine the best. Most crucially, the form in which a vitamin or mineral comes impacts how easily our bodies absorb it, which is known as bioavailability.

There’s a difference, too, in what else is in the products other than active ingredients – what we call additives and fillers.

If you’ve read any of my articles before, you’ll know I don’t subscribe to the view that all ultra-processed foods are bad. In the same way, not all additives and fillers are bad. Often, they serve a valid purpose.  

For instance, the daily recommended dose for vitamin D is 10 micrograms, which is such a tiny amount it would be practically impossible to consume without a bulking agent or filler. Similarly, an oil-based supplement needs an antioxidant included to stop it going rancid and to improve its shelf-life. 

However, some fillers are better than others, and we account for this in our scoring, based on the latest available evidence.

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See the results from our independent reviews of the best magnesium supplements and best multivitamins

How our tests find you the best

Most recently, we've reviewed magnesium supplements.

Although some are made with more bioavailable forms of magnesium, others fall short - and it's not always easy to tell the difference just by looking at the label. We've also discovered some big brand names that have lots of unnecessary additives or contain forms of magnesium that aren't easily absorbed by the body. That's where Which? steps in to help.

Our supplement assessment considers:

• Bioavailability of nutrients How easily absorbed the magnesium used is. Some formats are more readily absorbed by the body than others and have fewer side effects.
• Dose We assess the dose included and deduct points for doses that are too low to be effective, or are excessively high.
• Additives We rate the amount and type of bulkers, fillers and additives in each supplement. The fewer unnecessary extras included, the better.

We have minimum requirements for any recommended products and use an independent lab facility to test top-scorers to ensure our recommended products contain what they say they do.

Log in or Join Which? for unrestricted access to our magnesium test results.

Not yet a member? You can still access plenty of our health advice for free, including:

• Do you need to worry about fillers and bulkers in supplements?
• Five things our magnesium expert would never do
• Are you supplementing safely?
• Probiotics and gut health: what you need to know