Which Supplements Work Together, and Which Ones Don't

People in general, and I include myself in this to a degree, have a difficult time figuring out which supplements work together and which ones don't, so the question that brings more people to this website than almost any other isn't "what's the best nootropic" or "does lion's mane actually work". It's some version of "can I take NMN and astaxanthin together?" Google has shown our answer to that one well over a thousand times in the last three months, which tells me two things. People are building stacks, and hardly anyone is telling them how the pieces fit.

So the short answer first and the detail after. A handful of supplements help each other along, a few quietly undo each other, and a surprising amount comes down to what you take them with and when. Vitamin D wants a meal with some fat in it. Iron likes vitamin C and does not like your cup of tea. Calcium and iron shouldn't share a meal. Caffeine gets on well with L-theanine and badly with more caffeine. And anything you take alongside prescription medication is a question for your pharmacist rather than a blog, this one included.

The pairings that work better together

Vitamin D and a proper meal. Vitamin D is fat-soluble, which in plain terms means it needs some fat around to get absorbed well. Bess Dawson-Hughes and colleagues at Tufts tested this directly: fifty adults over 50 took the same dose of vitamin D3 with either a fat-free meal or one containing fat, and blood levels of D3 peaked 32% higher in the group who had the fat (Journal of the Academy of Nutrition and Dietetics, 2015). It was a single-dose study in older adults, so not the last word on the subject, but it costs you nothing to act on. If you've been taking your vitamin D first thing with a glass of water, move it to lunch or dinner, or take it alongside your krill oil, which brings its own fat to the party.

This matters more in Ireland than in most places. Your skin can only make vitamin D here from roughly late March to late September, so for the next five months most of us are running on whatever we eat and whatever we take. Our Vitamin D3 3,000iu is a once-a-day capsule, and dinner is a perfectly good time to take it.

Vitamin D and K2. These two get sold together a lot, and there's a sensible logic to it beyond marketing. Vitamin D contributes to the normal absorption of calcium, and both vitamin D and vitamin K contribute to the maintenance of normal bones, so if you're taking a decent dose of D through the winter, adding K2 MK7 is a reasonable pairing. One caution I'd put in bold if I could get away with it: vitamin K plays a part in normal blood clotting, so if you're on warfarin or any other blood thinner, don't add K2 without talking to your doctor first.

Vitamin C and iron. This is one of the few combinations with an EU-authorised claim sitting behind it: vitamin C increases iron absorption. If your GP has put you on iron, take it with vitamin C, whether that's a glass of orange juice or a capsule like our Vitamin C from natural extracts. And be especially careful if you have hemochromatosis! Vitamin C will turbo charge the amount of iron you're absorbing, even quicker than that faulty gene of yours alone! (don't worry, I'm allowed say that as I have hemochromatosis too.)

Caffeine and L-theanine. L-theanine is the amino acid in tea that gets most of the credit for why a cup of tea feels calmer than an espresso with a similar hit of caffeine. Put the two together deliberately and you have one of the most studied pairings in the whole nootropics space, with the theanine taking the jittery edge off the caffeine for a lot of people. I went through that evidence in Do Nootropics Actually Work?, and Smart Caffeine is the pairing in a single capsule.

NMN and astaxanthin. The one everyone searches for. The short answer is yes, and there's a reason our NMN comes with astaxanthin in the same capsule. The long answer, including how and when to take them, is in Can You Take NMN and Astaxanthin Together?

The ones to keep apart

Iron and your cup of tea. This is the bit for the Irish reader, because we are a nation that takes its tablets with tea. In 1999 Richard Hurrell's group measured how much iron people absorbed from a bread meal served with different drinks. Black tea cut absorption by between 79% and 94% compared with water, coffee and cocoa weren't far behind, and adding milk made little or no difference (British Journal of Nutrition, 1999). Those were single meals in a lab rather than months of normal life, and they measured the non-haem iron you get from plants and supplements rather than from meat. Even so, if you're taking iron because your bloods came back low, washing it down with a mug of builder's tea is close to pouring it down the sink. Leave an hour either side. I'm aware that may be the most upsetting sentence ever published on an Irish website.

Calcium and iron. Calcium competes with iron for absorption. A 1991 study in the American Journal of Clinical Nutrition found that calcium supplements taken with food reduced the absorption of both the iron in the meal and an iron supplement taken alongside it. If you take both, split them across the day, one in the morning and the other in the evening.

High-dose zinc, long term. Zinc and copper share an absorption route, and taking a lot of zinc for a long time can drag your copper levels down. That's a big part of why the EU's tolerable upper intake for zinc is set at 25mg a day. A normal zinc supplement through cold season is nowhere near that line. Taking three products that each contain zinc without realising it is how people get there.

Doubling up without knowing it. This is the most common problem of the lot, and it comes from the same ingredient quietly turning up in three different tubs. A multivitamin, an immune blend and a zinc tablet can easily triple your zinc. A pre-workout, a focus capsule and your third coffee of the morning can take you past the 400mg of caffeine a day that EFSA considers safe for healthy adults. Read the backs of the labels before you add something new rather than after, and if a label won't tell you how much of each ingredient is in there, that's worth knowing too (more on that in our red flags guide).

Timing: what goes in the morning and what goes at night

Most supplements don't much care what time it is. A few do.

Morning is for anything containing caffeine, for obvious reasons, and I'd put B vitamins there too, mainly because some people find a B complex a bit lively late in the day. Vitamin D goes with whichever meal has the most fat in it, which for most of us is lunch or dinner rather than a slice of toast at seven in the morning.

Evening belongs to magnesium. Magnesium contributes to normal psychological function and to the normal functioning of the nervous system, and I take MagTech at night myself. What I notice on it is that I wind down properly and sleep deeply, which is one man's experience rather than a finding, so take it as that. If you want the detail on which form of magnesium does what, it's all in Magnesium Glycinate vs Citrate vs L-Threonate.

Creatine is the one people overthink. When you take it barely matters; taking it every day matters a lot, as I covered in the creatine guide.

Supplements and prescription medication

This is where I'd stop reading blogs and ring a pharmacist. A few well-known examples: vitamin K and blood thinners, as above. Berberine, which affects several of the liver enzymes that process common medicines (there's more on that in Berberine vs GLP-1 Medications). And St John's wort, which we don't sell, but which interacts with a long list of drugs including the contraceptive pill. Your pharmacist can check your exact combination in a couple of minutes, for nothing, and would much rather you asked than guessed.

Building a stack without overthinking it

If all of this makes stacking sound like a chemistry exam, it really isn't. Add one thing at a time and give it a few weeks before you add the next, so you can tell what's doing what. Check the labels for overlap. Fat-soluble vitamins with food, stimulants early, magnesium late, and iron nowhere near the teapot. That covers most people most of the time.

Everything mentioned above is stocked and shipped from Dublin, and if you order by 4pm it goes out the same day.

Now put the kettle on. Just keep it away from the iron.

Frequently asked questions

Can I take vitamin D and magnesium together?

Yes. There's no conflict between them, and if your main meal is in the evening, taking both with dinner suits the two of them: vitamin D gets the fat it absorbs best with, and magnesium lands in the evening slot most people prefer.

Can I take zinc and iron at the same time?

At normal supplement doses it's better to separate them, because they compete for absorption. Take one in the morning and the other later in the day.

Should I take vitamin D in the morning or at night?

The time of day matters less than the meal. Vitamin D absorbs better with food that contains some fat, so take it with your most substantial meal, whenever that is.

Does tea stop supplements from working?

Tea doesn't affect most supplements, but it does markedly reduce how much plant and supplement iron you absorb, and milk doesn't fix it. If you take iron, leave about an hour between it and your tea or coffee.

How many supplements is too many?

There's no magic number. The bigger risk is overlap, where the same ingredient turns up in several products, so add one product at a time, read the labels, and ask your pharmacist if you take any prescription medication.

This article is for general information only and is not medical advice. If you are pregnant, breastfeeding, taking medication or managing a health condition, speak to your GP or pharmacist before starting any supplement.