Does low iron cause restless legs and muscle cramps?

For restless legs, iron helps, and by a modest amount. A Cochrane review of 7 placebo-controlled trials in 345 adults with restless legs found iron lowered severity by 3.78 points on the 40-point IRLS scale, measured 2 to 12 weeks after treatment. Whether low iron causes the condition is a separate question that the trials do not answer. For muscle cramps we found no randomized trial and no review of iron at all.

Established. In people diagnosed with restless legs syndrome, iron lowers symptom severity against placebo. The Cochrane review rated this moderate certainty, and a 2025 meta-analysis of 12 trials in 511 patients found a larger drop of 5.28 points. These are trials of iron supplements and of intravenous ferric carboxymaltose, a prescription drip, not of iron from food.

What the trials found

Severity of the legs

The Cochrane result of 3.78 points had a confidence interval from 1.31 to 6.25 and high heterogeneity between trials, and every trial carried at least one unclear risk of bias. The 2025 meta-analysis, which sits outside our local corpus and which we read as an abstract only, found iron lowered IRLS scores by 5.28 points (95% CI 2.90 to 7.66) and doubled the number of patients whose score improved (risk ratio 2.06). That abstract gives no baseline rate, so we cannot say how many more people out of 100 improved. A 2024 meta-analysis of 7 studies in 539 people found intravenous ferric carboxymaltose lowered scores by 5.77 points (95% CI 2.70 to 8.85).

Sleep did not follow

In the same Cochrane review, subjective sleep quality did not differ between iron and placebo (standardized difference 0.19, 95% CI -0.18 to 0.56, 3 studies, 128 people). Objective sleep efficiency, measured in a single study of 18 people, did not differ either. These secondary outcomes rest on few small trials and were rated low to very low certainty.

Single trials

In 110 people with moderate to severe restless legs and low iron stores without anemia, 1,000 mg of intravenous iron missed its main target at 4 weeks (difference -2.5 points, 95% CI -5.93 to 1.02). By week 12 the gap was 4.66 points and significant, on a secondary end point. In a very small trial of 29 people who also had iron-deficiency anemia, intravenous iron lowered scores by 13.47 points at 6 weeks against a rise of 1.36 on placebo. In 30 people with ferritin of 15 to 50 ng/mL, 12 weeks of iron tablets lowered scores by 9.1 points and the drug pramipexole by 8.7, a similar result. That was a small open comparison with no placebo arm.

Does low iron cause it

Unsettled. The link is real in observational data, and cause is not shown. Across 22 studies of 17,580 pregnant women in middle-income countries, low hemoglobin went with 68 percent higher odds of restless legs (adjusted odds ratio 1.68), and taking preventive iron with lower odds (0.59). Those are odds, not risks. In 113 untreated people whose ferritin sat inside the normal lab range, lower ferritin still predicted more severe symptoms, in a single-center cross-sectional study. Against that, a pooled analysis of brain MRI studies found no significant overall difference in brain iron. In the Cochrane review, trials that required low ferritin to enrol showed no significant difference from trials that did not, and oral iron showed none from intravenous. With about ten small trials, a missing subgroup difference is weak evidence either way.

Muscle cramps

Unsettled. We searched our corpus, Europe PMC and the web for trials or reviews of iron for muscle cramps and found no trial and no review, only case reports and symptom surveys. The one number that touches cramps is observational: in the same pregnancy studies, women with muscle cramps had higher odds of restless legs (adjusted odds ratio 3.58, 95% CI 1.21 to 10.61). That says the two complaints travel together in pregnancy. It says nothing about whether iron prevents cramps, because nobody we found has tested it.

Who should be careful

Iron brings side effects. In the 2025 meta-analysis, any adverse event was about twice as common on iron (risk ratio 2.04, 95% CI 1.46 to 2.85). Serious adverse events did not differ significantly (risk ratio 2.04, 95% CI 0.39 to 10.81), and neither did stopping treatment. An earlier meta-analysis of 10 trials found the same doubling, with side effects that were not severe and did not make more people stop.

Not for everyone. Some people with restless legs take levodopa. The NIH notes that iron supplements reduce levodopa absorption in healthy people, possibly by binding it, and levodopa labels in the United States carry this warning. The NIH gives no size for the effect. Separately, observational data across 907 patients in 6 studies link low ferritin to augmentation, the worsening of restless legs on dopamine drugs. The abstract reports a p-value and no effect size.

The NIH sets the upper limit for iron at 45 mg a day for adults and 40 to 45 mg for infants, children and teenagers. The doses in the trials above were prescribed after blood tests of iron status, which is what the guideline below asks for.

What expert bodies say

The American Academy of Sleep Medicine, in its 2025 guideline, advises testing ferritin and transferrin saturation in restless legs. For adults it suggests oral or intravenous iron when ferritin is 75 ng/mL or lower or transferrin saturation is under 20 percent, and intravenous iron only when ferritin is between 75 and 100 ng/mL. The guideline itself says these thresholds are consensus and have not been tested empirically. It strongly recommends intravenous ferric carboxymaltose for adults with suitable iron status, on moderate certainty evidence, and conditionally suggests oral ferrous sulfate.

How we searched

What would change this answer

• Any randomized trial of iron for muscle cramps. On this half of the question there is nothing to weigh.
• Trials that test the ferritin thresholds in the AASM guideline. They are consensus, and the Cochrane review found no clear difference between trials that did and did not require low ferritin.
• Larger trials of iron tablets, as opposed to infusions, with sleep measured as an outcome. The current trials are small, mostly intravenous, and found no change in sleep quality.
• Results from ClinicalTrials.gov. One completed trial of sucrosomial iron in restless legs has results posted in the registry, which we did not read for this page.

How much iron you need and where it comes from in food is in the iron guide. Iron for anemia, a separate question, has its own evidence page.

Sources

1. ev-irrls-01 · Meta-analysis or systematic review · Cochrane systematic review and meta-analysis of randomized trials · n = 345
   Combining data from the seven trials using the IRLS to compare iron and placebo, use of iron resulted in greater improvement in IRLS scores (MD -3.78, 95% CI -6.25 to -1.31; I2= 66%, 7 studies, 345 participants) measured 2 to 12 weeks after treatment.
   
   Who: adults diagnosed with restless legs syndrome in controlled trials of oral or intravenous iron, followed 2 to 16 weeks
   Effect: IRLS MD -3.78 (95% CI -6.25 to -1.31; I2 66%; 7 studies, 345 participants); with an eighth study SMD -0.74 (-1.26 to -0.23); GRADE moderate
   Certainty: GRADE moderate for the primary outcome; every included trial had at least one unclear risk of bias; high heterogeneity.
   Cochrane Database Syst Rev, 2019 · checked 2026-10-06 · we read the abstract
2. ev-irrls-02 · Meta-analysis or systematic review · Cochrane systematic review and meta-analysis of randomized trials · n = 128
   Subjective sleep quality was no different between iron and placebo groups (SMD 0.19, 95% CI -0.18 to 0.56; I2 = 9%, 3 studies, 128 participants), nor was objective sleep quality, as measured by change in sleep efficiency in a single study (-35.5 +/- 92.0 versus -41.4 +/- 98.2, 18 participants).
   
   Who: adults with restless legs syndrome in trials of iron versus placebo
   Effect: subjective sleep quality SMD 0.19 (95% CI -0.18 to 0.56; 3 studies, 128 participants); periodic limb movements SMD -0.19 (-0.70 to 0.32; 2 studies, 60 participants)
   Certainty: Few small trials per secondary outcome; GRADE low to very low.
   Cochrane Database Syst Rev, 2019 · checked 2026-10-06 · we read the abstract
3. ev-irrls-03 · Meta-analysis or systematic review · Cochrane systematic review and meta-analysis of randomized trials, subgroup analysis · n = 428
   The use of low serum ferritin levels as an inclusion criteria and the use or oral versus intravenous iron did not show significant subgroup differences.
   
   Who: adults with restless legs syndrome in trials of iron versus placebo, prespecified subgroups
   Effect: larger improvement in dialysis trials; no significant subgroup difference by low-ferritin inclusion criterion or by oral vs intravenous route
   Certainty: Subgroup analyses across 10 small trials; absence of a subgroup difference is not proof the groups respond the same.
   Cochrane Database Syst Rev, 2019 · checked 2026-10-06 · we read the abstract
4. ev-irrls-04 · Meta-analysis or systematic review · systematic review and meta-analysis of RCTs · n = 511
   Twelve randomized controlled trials with 511 patients were included. Iron improved the IRLS score (mean difference [MD] = -5.28; 95% confidence interval [CI] - 7.66, -2.90; P < 0.0001), the number of patients with improved IRLS score (risk ratio [RR] = 2.06; 95% CI 1.49, 2.84; P < 0.0001), RLS-QOL (MD = 7.42; 95% CI 1.32, 13.51; P = 0.02), and sleep VAS score (MD = -24.83; 95% CI -40.08, -9.58; P = 0.001), but yielded more overall AEs (RR = 2.04; 95% CI 1.46, 2.85; P < 0.0001), with no difference in serious AEs or those leading to drug discontinuation (RR = 2.04; 95% CI 0.39, 10.81; P = 0.40 and RR = 6.25; 95% CI 0.79, 49.54; P = 0.08), respectively.
   
   Who: patients with restless legs syndrome in randomized controlled trials of iron supplements versus placebo
   Effect: IRLS MD -5.28 (95% CI -7.66 to -2.90); improved IRLS RR 2.06 (1.49 to 2.84); RLS-QOL MD 7.42 (1.32 to 13.51); overall adverse events RR 2.04 (1.46 to 2.85); serious adverse events RR 2.04 (0.39 to 10.81)
   Certainty: Newest synthesis (2025), outside the local corpus snapshot; abstract only (PMC full text did not open). Ferric carboxymaltose trials dominate.
   Proc (Bayl Univ Med Cent), 2025: Efficacy and safety of iron supplements for restless leg syndrome, a systematic review and meta-analysis · checked 2026-10-06 · we read the abstract
5. ev-irrls-05 · Meta-analysis or systematic review · systematic review and meta-analysis of RCTs · n = —
   Iron was associated with an increased rate of adverse events RR 2.04 (95% CI 1.46-2.85), which were not severe and not associated with increased rate of treatment discontinuation.
   
   Who: patients with restless legs syndrome in randomized controlled trials of iron versus no iron
   Effect: IRLS WMD -3.55 (95% CI -5.41 to -1.68); improvement RR 2.16 (1.56 to 2.98); adverse events RR 2.04 (1.46 to 2.85), not severe, no rise in discontinuation
   Certainty: The abstract does not give the total number of participants.
   Eur J Intern Med, 2019 · checked 2026-10-06 · we read the abstract
6. ev-irrls-06 · Meta-analysis or systematic review · systematic review and meta-analysis · n = 539
   Seven studies involving 539 participants were analyzed. FCM significantly reduced IRLS scores (WMD = -5.77; 95% CI = [-8.85, -2.70]; p = 0.0002) and improved VAS and SF-36 scores compared to placebo.
   
   Who: patients diagnosed with restless legs syndrome given intravenous ferric carboxymaltose
   Effect: IRLS WMD -5.77 (95% CI -8.85 to -2.70); RLS quality of life not significantly improved; more adverse events, mainly nausea, no difference in severe adverse events
   Certainty: Registered in PROSPERO; studies of intravenous iron only, not dietary iron.
   Front Neurol, 2024 · checked 2026-10-06 · we read the abstract
7. ev-irrls-07 · Randomized controlled trial(s) · randomized placebo-controlled multicentre trial · n = 110
   Ferric carboxymaltose treatment (n = 59) led to nonsignificant improvement over placebo (n = 51) in International Restless Legs Syndrome Severity Scale score at week 4 (difference [95% confidence interval], -2.5 [-5.93 to 1.02], P = 0.163), reaching significance by week 12 (-4.66 [-8.59 to -0.73], P = 0.021).
   
   Who: patients with moderate to severe restless legs syndrome and serum ferritin < 75 ug/L (or 75-300 ug/L with transferrin saturation < 20%), no anemia
   Effect: IRLS difference week 4 -2.5 (95% CI -5.93 to 1.02, P = 0.163); week 12 -4.66 (-8.59 to -0.73, P = 0.021)
   Certainty: Primary end point at week 4 missed; the week 12 benefit is a secondary end point.
   Mov Disord, 2017 · checked 2026-10-06 · we read the abstract
8. ev-irrls-08 · Randomized controlled trial(s) · randomized double-blind placebo-controlled trial · n = 29
   At week six post-infusion, FCM compared to placebo group showed significant improvement from baseline in IRLS score (-13.47 ± 7.38 vs. 1.36 ± 3.59).
   
   Who: patients with restless legs syndrome and iron deficiency anemia
   Effect: IRLS change at week 6: -13.47 +/- 7.38 (iron) vs +1.36 +/- 3.59 (placebo); 61% of responders off RLS medication at week 52
   Certainty: Very small trial (15 vs 14); long-term phase followed responders only.
   Sleep Med, 2021 · checked 2026-10-06 · we read the abstract
9. ev-irrls-09 · Randomized controlled trial(s) · randomized comparative trial · n = 30
   After 12 weeks, IRLS scores were lower than those at baseline in both groups (iron -9.1 ± 7.07, P < 0.001; pramipexole -8.7 ± 8.31, P = 0.001) and similar between the two groups.
   
   Who: restless legs syndrome patients with low-normal serum ferritin (15-50 ng/ml)
   Effect: IRLS change at 12 weeks: iron -9.1 +/- 7.07 vs pramipexole -8.7 +/- 8.31; response (>=50% fall) 46.7% in both groups
   Certainty: Small open comparison against an active drug, no placebo arm.
   Eur J Neurol, 2014 · checked 2026-10-06 · we read the abstract
10. ev-irrls-10 · Observational data · systematic review and meta-analysis of observational studies · n = 17580
    The overall pooled prevalence of RLS among pregnant women in middle-income countries was 13.82% (95% CI: 13.31, 14.32), and having low hemoglobin level (AOR: 1.68, 95% CI: 1.29, 2.18), history of RLS (AOR: 7.54, 95% CI: 3.02, 18.79), muscle cramps (AOR: 3.58, 95% CI: 1.21, 10.61), excessive day time sleepiness (AOR: 4.02, 95% CI: 1.34, 12.04), preeclampsia (AOR: 2.06, 95% CI: 1.28, 3.30), and taking prophylactic iron supplementation (AOR: 0.59, 95% CI: 0.50, 0.69) were the identified factors associated with it.
    
    Who: pregnant women in middle-income countries, 22 studies from nine countries
    Effect: pooled RLS prevalence 13.82% (95% CI 13.31 to 14.32); low hemoglobin AOR 1.68 (1.29 to 2.18); muscle cramps AOR 3.58 (1.21 to 10.61); prophylactic iron AOR 0.59 (0.50 to 0.69)
    Certainty: Observational associations; odds ratios, not risks; cannot show iron prevents RLS.
    Front Med (Lausanne), 2023 · checked 2026-10-06 · we read the abstract
11. ev-irrls-11 · Observational data · systematic review and meta-analysis of observational studies · n = 907
    The results revealed that augmented RLS was significantly associated with low serum ferritin levels (p = 0.002), high LEDs (p = 0.026), and nonsignificantly associated with high IRLS scores (p = 0.227).
    
    Who: restless legs syndrome patients with and without augmentation on dopaminergic treatment, 6 observational studies
    Effect: augmented RLS significantly associated with low serum ferritin (p = 0.002); 220 augmented vs 687 non-augmented patients
    Certainty: Observational; the abstract gives a p-value but no pooled effect size.
    Sleep Med, 2023 · checked 2026-10-06 · we read the abstract
12. ev-irrls-12 · Observational data · cross-sectional study · n = 113
    Multiple regression analysis identified serum ferritin level (p < 0.001) and age (p = 0.001) as independent and significant predictors of RLS severity.
    
    Who: drug-naive patients with idiopathic restless legs syndrome and serum ferritin within the reference range (22-322 ug/L)
    Effect: serum ferritin an independent predictor of RLS severity (p < 0.001), median ferritin 51.4 ug/L
    Certainty: Single center, cross-sectional; cannot show that raising ferritin lowers severity.
    Sleep Med, 2026 · checked 2026-10-06 · we read the abstract
13. ev-irrls-13 · Observational data · case-control MRI study with meta-analysis of imaging studies · n = 144
    The meta-analysis revealed no significant pooled effect across all brain regions.
    
    Who: 72 RLS patients and matched controls, plus meta-analysis of earlier iron-sensitive MRI studies
    Effect: no significant pooled effect across brain regions; increased iron signals in caudate, putamen and red nucleus in the new cohort
    Certainty: Imaging proxies for iron content; mechanism question still open.
    Neuroimage Clin, 2022 · checked 2026-10-06 · we read the abstract
14. ev-irrls-14 · Position of an expert body · clinical practice guideline (GRADE) · n = —
    Consensus guidelines, which have not been empirically tested, suggest that supplementation of iron in adults with RLS should be instituted with oral or IV iron if serum ferritin ≤ 75 ng/mL or transferrin saturation < 20%, and only with IV iron if serum ferritin is between 75 and 100 ng/mL.
    
    Who: adults and children with clinically significant restless legs syndrome
    Effect: adults: oral or IV iron if ferritin <= 75 ng/mL or transferrin saturation < 20%; IV only if ferritin 75-100 ng/mL; children: ferritin < 50 ng/mL
    Certainty: The guideline itself says these thresholds are consensus and not empirically tested.
    J Clin Sleep Med, 2025 · checked 2026-10-06 · we read the abstract
15. ev-irrls-15 · Position of an expert body · clinical practice guideline (GRADE) · n = —
    In adults with RLS, the AASM recommends the use of IV ferric carboxymaltose over no IV ferric carboxymaltose in patients with appropriate iron status (see good practice statement for iron parameters) (strong recommendation, moderate certainty of evidence).
    
    Who: adults with restless legs syndrome and appropriate iron status
    Effect: IV ferric carboxymaltose: strong recommendation, moderate certainty; ferrous sulfate: conditional recommendation, moderate certainty
    Certainty: Guideline grades; recommendations depend on iron status thresholds in card 14.
    J Clin Sleep Med, 2025 · checked 2026-10-06 · we read the abstract
16. ev-irrls-16 · Position of an expert body · agency fact sheet · n = —
    Some evidence indicates that in healthy people, iron supplements reduce the absorption of levodopa (found in Sinemet and Stalevo), used to treat Parkinson’s disease and restless leg syndrome, possibly through chelation [101-103].
    
    Who: General population, NIH Office of Dietary Supplements iron fact sheet
    Effect: iron supplements reduce levodopa absorption, possibly by chelation; label warning in the United States
    Certainty: Agency summary without interaction size; relevant because some people with RLS take levodopa.
    NIH Office of Dietary Supplements, Iron fact sheet for health professionals · checked 2026-10-06 · we read the section
17. ev-irrls-17 · Position of an expert body · agency fact sheet · n = —
    The Tolerable Upper Intake Level for iron is 45 mg for adults, and it ranges from 40 mg to 45 mg for infants, children, and adolescents, depending on age.
    
    Who: General population, NIH Office of Dietary Supplements iron fact sheet
    Effect: UL 45 mg/day adults, 40-45 mg/day for infants, children and adolescents
    Certainty: UL is set on gastrointestinal effects of iron salts; therapeutic RLS doses in trials are prescribed after iron tests (AASM, card 14).
    NIH Office of Dietary Supplements, Iron fact sheet for health professionals · checked 2026-10-06 · we read the section