Quick answer

It depends on the use. Liquid magnesium citrate sold as a laxative generally works within half an hour to 6 hours. As a daily supplement, blood magnesium can rise within hours of a dose but keeps building for weeks, leveling off after about 20 weeks in pooled trials. The sleep trial of bisglycinate ran four weeks.^1,2,3,4

Key facts

After one 400 mg dose of citrate, blood magnesium was higher than at the start at every check from 2 to 24 hours.²
Magnesium in urine, a marker of how much is absorbed, kept rising for about 40 weeks of daily use in pooled trials.³
In the bisglycinate sleep trial, scores moved in both the magnesium and placebo groups during the first two weeks.⁴
The usual blood test says little about the body's total magnesium, so it cannot show how full your stores are.⁵

As a laxative: within hours

Magnesium citrate sold as a saline laxative generally produces a bowel movement within half an hour to 6 hours, according to its label.¹

That speed comes from magnesium the gut does not absorb, which draws water into the bowel. A laxative dose of 6.5 to 10 fluid ounces holds far more magnesium than a supplement, and the label limits it to occasional use.^1,5

After one supplement dose: hours

In 14 men who had taken magnesium for five days beforehand, one 400 mg dose of magnesium citrate raised blood magnesium at every check from 2 to 24 hours, while the same dose of magnesium oxide did not.²

In a 2024 study funded by the maker of a competing form, 40 adults on a low-magnesium diet took single doses after an overnight fast. Blood magnesium rose significantly at one point after oxide and after citrate, but not at any point in the 6 hours after bisglycinate.⁶

A rise in blood magnesium after a dose shows that some was absorbed. It is not the same as an effect you would notice, and neither study measured sleep, mood or energy.^2,6

With daily use: weeks to months

In a pooled analysis of 48 trials, blood magnesium rose gradually with daily supplements and leveled off after about 20 weeks; magnesium in urine leveled off after about 40 weeks.³

The trials in that analysis ran from 3 weeks to 5 years, with a median of 12 weeks. People who started with higher blood magnesium saw a smaller rise, so someone already well supplied may see little change at all.³

The usual blood test measures serum magnesium, which says little about how much magnesium the body holds overall, and no single test is considered satisfactory.⁵

For sleep: four weeks in the trial

May modestly improve self-rated sleep in adults who sleep poorly*^4,7

Early evidence

One randomized, placebo-controlled trial tested this form: 155 adults who slept poorly took magnesium bisglycinate (250 mg of magnesium a day) or placebo for four weeks. Scores on a sleep-problem questionnaire fell a little more with magnesium (3.9 points against 2.3), a small effect, and other measures did not differ. It is a single short trial that relied on self-report. Older trials used other forms at higher doses in older adults and were rated low quality.

Amount studied: 250 mg a day as bisglycinate, 30 to 60 minutes before bed, for four weeks

In that trial, scores on the sleep questionnaire fell in both groups within the first two weeks. By day 28 they were 28% lower with magnesium and 18% lower with placebo. The trial stopped at four weeks, so it says nothing about longer use.⁴

The L-threonate trials lasted 21 days and six weeks. In the shorter one, the main sleep questionnaire improved in the placebo group too, so a change in the first weeks is not proof that the supplement caused it.^8,9

What's in ours

What’s in ours

Magnesium Glycinate: Magnesium, 275 mg per serving, from the label
Men's Multi: Magnesium, 50 mg per serving, from the label
Creatine + Electrolytes: Magnesium, 60 mg per serving, from the label

When not to wait it out

The magnesium citrate laxative label says to stop and ask a doctor if there is rectal bleeding or no bowel movement after use, and to ask first if you have kidney disease, are on a magnesium-restricted diet, or have used a laxative for more than a week.¹

If you take magnesium for sleep and problems are severe or have lasted for weeks, see a doctor rather than raising the dose past the 350 mg limit for supplements.¹⁰

Questions people ask

How long does magnesium glycinate take to work for sleep?

In the one trial of this form, people took it for four weeks. Scores fell in both groups within two weeks and ended slightly lower with magnesium, and the evidence is graded early.⁴

How long does magnesium oxide take to work?

As a supplement, little of it is absorbed: about 4% in one study. After a single dose, one study found no significant rise in blood magnesium over 24 hours, and another found a rise at the 1-hour check only.^2,6,11

How do I know if magnesium is working?

There is no simple test, because blood magnesium says little about the body's stores. Judge it by whatever you are taking it for, over the weeks the studies used.⁵

See what we make

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See the Full Measure line Creatine + Electrolytes Magnesium Glycinate Men's Multi

Sources

*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.

This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.