Ergocalciferol is a form of vitamin D (specifically vitamin D2) used to treat vitamin D deficiency and several conditions related to calcium and phosphorus imbalance. It’s available as both a prescription-strength capsule, typically at 50,000 IU, and as an over-the-counter liquid supplement at lower doses. The prescription form is FDA-approved for three specific conditions: hypoparathyroidism, refractory rickets, and familial hypophosphatemia.
Conditions It Treats
At prescription strength, ergocalciferol targets conditions where the body can’t properly regulate calcium or phosphorus on its own. In hypoparathyroidism, the parathyroid glands don’t produce enough hormone to maintain normal calcium levels. High-dose ergocalciferol (50,000 to 200,000 IU per day, alongside calcium supplements) helps the body absorb enough calcium from food to compensate. It’s the least expensive option for managing this condition and provides a long duration of action, which can mean fewer fluctuations in calcium levels compared to shorter-acting alternatives.
Refractory rickets, also called vitamin D-resistant rickets, doesn’t respond to the usual doses of vitamin D that would correct nutritional rickets. These patients may need anywhere from 12,000 to 500,000 IU per day. Familial hypophosphatemia, a genetic condition causing chronically low phosphorus levels in the blood, also requires high-dose treatment to support proper bone mineralization.
At lower, over-the-counter doses, ergocalciferol is used as a dietary supplement to correct or prevent vitamin D deficiency. Many doctors prescribe 50,000 IU once weekly for 8 weeks as a common repletion protocol for adults with significantly low vitamin D levels, then transition to a daily maintenance dose.
How It Works in Your Body
Ergocalciferol is derived from a plant compound called ergosterol, which is found in fungi and some plants. When you take it, your body converts it in two steps. First, the liver transforms it into a circulating form called 25-hydroxyvitamin D, which is what blood tests measure to check your vitamin D status. Then the kidneys convert that into the fully active hormone form, which regulates how much calcium and phosphorus your intestines absorb from food and how much your kidneys retain rather than excrete.
This two-step activation process means ergocalciferol doesn’t work instantly. It takes days to weeks of consistent dosing before blood levels reach their target, and because it’s stored in fat tissue, it also takes a long time to clear the body once you stop taking it.
Ergocalciferol vs. Cholecalciferol
Vitamin D comes in two supplemental forms: D2 (ergocalciferol) and D3 (cholecalciferol). D3 comes from animal sources like fish oil and lanolin, while D2 comes from plants and fungi. This makes ergocalciferol the go-to option for people following vegan or strict vegetarian diets.
The two forms go through the same metabolic pathway, but they aren’t equally potent. A systematic review and meta-analysis confirmed that D3 raises blood levels of vitamin D significantly more than D2 at equivalent doses. In one study of female healthcare workers, D3 supplementation increased blood levels by an average of 8.5 ng/mL over six months, compared to just 4.7 ng/mL with D2. Another finding: while D3 raised levels in all participants, D2 failed to do so in people who weren’t already deficient.
Despite this, ergocalciferol remains widely used because it’s inexpensive, it’s the form most commonly available in high-dose prescription capsules, and it’s effective enough for treating deficiency when dosed appropriately. For the FDA-approved conditions like hypoparathyroidism, where very high doses are used, the cost advantage is particularly meaningful.
Natural Sources of Vitamin D2
Very few foods naturally contain ergocalciferol. The richest source is mushrooms, particularly those exposed to ultraviolet light. White button mushrooms treated with UV light develop significant amounts of D2 and have been studied as a viable dietary source. Wild mushrooms that grow in sunlight also contain meaningful amounts. Some fortified foods like certain plant milks and cereals use D2 rather than D3, though this varies by brand.
Risks of Taking Too Much
Because ergocalciferol increases calcium absorption, the primary danger of overdose is too much calcium in the blood. Early symptoms include loss of appetite, nausea, vomiting, and constipation. As calcium levels climb higher, you may experience excessive thirst, frequent urination, dehydration, confusion, and muscle weakness. Severe toxicity can lead to kidney stones, kidney failure, and abnormal heart rhythms.
Toxicity is essentially impossible from food or sun exposure. It occurs from taking high-dose supplements over extended periods without monitoring. The long half-life of ergocalciferol is a double-edged sword here: the same property that makes it convenient for treating hypoparathyroidism (less frequent dosing, steadier levels) also means that if toxicity develops, it takes longer to resolve after you stop the supplement. Patients on high-dose regimens typically need periodic blood tests to check both vitamin D and calcium levels.
Medications That Interact With It
A few common medications can create problems when combined with ergocalciferol. Thiazide diuretics, often prescribed for high blood pressure, reduce the amount of calcium your kidneys excrete. Pairing them with vitamin D supplementation, which increases calcium absorption, can push blood calcium levels dangerously high. This risk is greatest in older adults and people with impaired kidney function.
On the other side, some medications reduce how well your body absorbs ergocalciferol. Weight-loss drugs that block fat absorption (like orlistat) can also block this fat-soluble vitamin. Bile acid sequestrants, used to lower cholesterol, may bind vitamin D in the gut and increase its excretion. If you take any of these medications, spacing your vitamin D dose or adjusting the amount may be necessary.