Last updated: October 2026
Many magnesium supplements are wasted because of the form, the dose or the person taking them. Cheap magnesium oxide is poorly absorbed, labels can confuse compound weight with actual magnesium, and if your diet already covers the NHS’s 270mg (women) or 300mg (men) a day, extra may make little difference. Choosing well avoids most of these.
Below: the seven most common reasons magnesium doesn’t work, how to read a label, and how to choose one that’s worth taking.
7 reasons magnesium supplements are wasted
- The form is poorly absorbed. Magnesium oxide is the cheapest form and very common on the high street. One study measured only about 4% of it being absorbed, and a 60-day University of Reading trial found oxide made no difference compared with placebo, while citrate and an amino acid chelate (the family glycinate belongs to) were absorbed better.
- It goes straight through you. High single doses, especially of oxide or citrate, draw water into the gut and cause loose stools, so less is absorbed. Splitting the dose or choosing a gentler form helps.
- The dose on the front isn’t the magnesium. “1,000mg magnesium bisglycinate” may contain only around 140mg of actual (elemental) magnesium. See how to read a label.
- You weren’t low to begin with. If your diet already provides enough, a supplement is unlikely to change how you feel. See signs of magnesium deficiency.
- Something is blocking it. Long-term use of acid-reducing medicines called proton pump inhibitors (such as omeprazole) can lower magnesium levels, according to the MHRA. Very high-dose zinc can also compete with magnesium.
- Taken at the wrong time alongside medicines. Magnesium reduces absorption of some antibiotics and osteoporosis medicines, and vice versa. Leave a gap of a few hours and ask a pharmacist. See best time to take magnesium.
- Expecting too much, too soon. Magnesium isn’t a sleeping pill. Most people need to take it consistently for a few weeks to judge. See how long magnesium takes to work and signs your magnesium is working.
How to read a magnesium label
UK labels must list the amount of the nutrient itself, with a %NRV (the daily reference value for magnesium is 375mg). Look in the nutrition table for “magnesium” in mg and %NRV, not the compound name in large print on the front.
| Compound | Roughly how much is actual magnesium |
|---|---|
| Magnesium oxide | About 60%, but poorly absorbed |
| Magnesium citrate | About 11 to 16%, depending on the type |
| Magnesium bisglycinate | About 14% |
| Magnesium L-threonate | About 7 to 8% |
So a high-percentage form isn’t automatically better: oxide carries lots of magnesium on paper, but little of it gets in.
Forms compared
| Form | Absorption | Stomach | Typical use |
|---|---|---|---|
| Glycinate (bisglycinate) | Good | Gentle | Everyday and evening use |
| Citrate | Good | Can loosen stools at higher doses | Everyday use; constipation |
| L-threonate | Good, but low magnesium per dose | Gentle | Marketed for the brain |
| Oxide | Poor | Most likely to cause diarrhoea | Cheap tablets; laxative use |
For the full comparison, see magnesium glycinate vs citrate vs threonate and types of magnesium.
Do you need a supplement at all?
The NHS says adults aged 19 to 64 need 300mg of magnesium a day (men) or 270mg (women), and most people can get this from a varied diet: nuts and seeds, leafy greens, beans, wholemeal bread and brown rice. It also says 400mg a day or less from supplements is unlikely to cause harm.
A supplement makes most sense if your diet is low in those foods, you train hard, or you simply want a consistent evening routine. Magnesium contributes to the reduction of tiredness and fatigue, normal muscle function and normal psychological function.
How to choose one that’s worth it
- A well-absorbed form, such as glycinate or citrate, not mainly oxide.
- Elemental magnesium stated clearly, with a %NRV.
- A sensible dose, well within 400mg a day from supplements, ideally split or taken with food if your stomach is sensitive.
- A format you’ll actually take every day: tablets, capsules or a powder drink.
We built our Magnesium Glycinate Sleep Complex around these points: bisglycinate and citrate with glycine, L-theanine and botanicals, giving 148mg of magnesium per serving as a warm evening drink. See also magnesium powder vs tablets and best magnesium supplement UK.
FAQs
Why are some magnesium supplements a waste of money?
Mainly because of poorly absorbed forms like oxide, confusing labels, doses that cause diarrhoea, or taking it when your diet already provides enough.
Is magnesium oxide any good?
It’s poorly absorbed, with one study measuring around 4%. It’s mainly useful as a laxative. Glycinate and citrate are absorbed better.
What is elemental magnesium?
The amount of actual magnesium in a compound. It’s what the nutrition table on UK labels shows, with a %NRV.
How do I know if my magnesium is working?
Give it a few weeks of daily use and notice tiredness, sleep and muscle cramps. If nothing changes, you may not have needed it.
Is magnesium citrate or glycinate better?
Both are well absorbed. Glycinate is gentler on the stomach; citrate can loosen stools at higher doses. Some products combine both.
How much magnesium should I take a day?
The NHS says adults need 270mg (women) to 300mg (men) a day, mostly from food, and 400mg or less from supplements is unlikely to cause harm.
Sources
- Firoz M, Graber M. Bioavailability of US commercial magnesium preparations. Magnes Res, 2001.
- Walker AF et al. Mg citrate found more bioavailable than other Mg preparations in a randomised, double-blind study. Magnes Res, 2003.
- MHRA. Proton pump inhibitors in long-term use: reports of hypomagnesaemia.
- NHS. Vitamins and minerals: magnesium.
