The standard oral iron dose studied for restless leg syndrome is 325 mg of ferrous sulfate taken twice a day, paired with 100 mg of vitamin C at each dose to aid absorption. That said, iron therapy for RLS only works when your iron stores are actually low, and the threshold for “low” in RLS is higher than what most standard blood tests flag as deficient.
Why Iron Levels Matter for RLS
Iron plays a direct role in producing dopamine, the brain chemical responsible for smooth, controlled movement and the suppression of unwanted sensory signals. Iron is required at the bottleneck step of dopamine production. When iron stores in the brain drop, dopamine output falls with it, and the uncomfortable urge to move your legs intensifies, especially at night when dopamine levels naturally dip.
This is why iron supplementation can reduce or eliminate RLS symptoms in people whose iron stores are below optimal levels. It’s also why iron does nothing for people whose stores are already adequate. The treatment is correcting a deficiency, not adding a therapeutic drug.
The Ferritin Numbers That Guide Treatment
Your ferritin level, a blood marker of stored iron, determines whether iron supplementation is appropriate and which form you should take. The American Academy of Sleep Medicine (AASM) guidelines break it down clearly:
- Ferritin below 75 ng/mL or transferrin saturation below 20%: Oral iron (ferrous sulfate) or IV iron is recommended.
- Ferritin between 75 and 100 ng/mL: IV iron only, because oral iron loses effectiveness as ferritin climbs. Your body’s absorption-regulating system, driven by a hormone called hepcidin, increasingly blocks iron uptake from the gut once stores reach this range.
- Ferritin above 100 ng/mL with transferrin saturation above 20%: Iron supplementation is unlikely to help. These are considered optimal iron levels for RLS patients.
That 100 ng/mL target is worth noting. A general lab report might flag ferritin as “normal” at 30 or 40 ng/mL. For the general population, that is normal. For someone with RLS, it’s low enough to drive symptoms. If your doctor has told you your iron levels are fine but hasn’t looked at them through the lens of RLS-specific thresholds, it’s worth revisiting.
Oral Iron: Dose and How to Take It
The dose used across multiple clinical trials is ferrous sulfate 325 mg, taken twice daily. Each 325 mg tablet contains about 65 mg of elemental iron (the actual iron your body can use), so you’re getting roughly 130 mg of elemental iron per day split across two doses.
Expert consensus guidelines recommend pairing each dose with 100 mg of vitamin C. Vitamin C creates a more acidic environment in the stomach that helps iron dissolve and stay in its most absorbable form. The effect is modest when iron is taken with a full meal, so taking it on an empty stomach or with only a small amount of food gives you the best absorption. Common advice is to take iron one hour before or two hours after eating, though some people find this hard on the stomach.
The typical trial period is 12 weeks. RLS symptom improvement tends to be gradual rather than dramatic, and you may not notice meaningful changes for several weeks. If ferritin hasn’t risen meaningfully after three months of oral supplementation, or if you can’t tolerate the side effects, IV iron becomes the next step.
Side Effects and Tolerance
Iron supplements are notorious for causing stomach upset, nausea, constipation, and dark stools. These side effects are the main reason people stop taking them. A few strategies can help. Taking iron with a small amount of food reduces nausea, though it also reduces absorption somewhat. Some people tolerate liquid iron formulations better than tablets. If twice-daily dosing is too harsh on your gut, every-other-day dosing has been shown in broader iron research to maintain reasonable absorption while cutting side effects significantly, though this specific schedule hasn’t been tested in RLS trials.
When IV Iron Makes More Sense
IV iron bypasses the gut entirely, which solves two problems at once: it avoids gastrointestinal side effects, and it delivers iron directly into the bloodstream without relying on intestinal absorption. The AASM gives its strongest recommendation to a specific IV formulation (ferric carboxymaltose) for patients with ferritin below 100 ng/mL or transferrin saturation below 20%.
IV iron is specifically preferred over oral iron in three situations: when ferritin is between 75 and 100 ng/mL (where oral iron has minimal benefit due to reduced gut absorption), when oral iron causes intolerable side effects, or when rapid symptom relief is needed. A single IV infusion can take as little as 15 minutes and often produces noticeable improvement faster than weeks of oral supplementation.
Get Tested Before You Supplement
Taking iron without knowing your baseline levels carries real risk. Iron overload can damage the liver, heart, and other organs, and some people carry genetic variants for hemochromatosis (a condition that causes the body to absorb too much iron) without knowing it. The AASM recommends that every RLS patient have their ferritin and transferrin saturation checked before starting any iron therapy. Ideally, blood should be drawn in the morning, and you should avoid iron-containing supplements and iron-rich foods for at least 24 hours before the test to get an accurate reading.
Once you’re on iron, ferritin and transferrin saturation should be rechecked once or twice a year. If your transferrin saturation climbs above 45%, that’s a signal to stop supplementation and investigate further. Anyone with a personal or family history suggestive of hemochromatosis or unexplained high iron markers should be evaluated before treatment begins.
Iron Won’t Help Everyone With RLS
Iron therapy targets one specific mechanism of RLS: insufficient iron stores feeding into reduced dopamine production. If your ferritin is already above 100 ng/mL and your transferrin saturation is above 20%, adding more iron won’t improve your symptoms and could cause harm. In those cases, RLS is likely being driven by other factors, and different treatments are more appropriate. The blood test is the essential first step that determines whether iron is your answer or a dead end.