Vitamin D2 (ergocalciferol) is one of the two main forms of vitamin D used in supplements and medicine. It comes from plant and fungal sources — specifically from ergosterol, a compound found in mushrooms and yeast that converts to D2 when exposed to UV light. Unlike vitamin D3 (which comes from animal sources like fish oil or lanolin), D2 is fully vegan and has been the form used in pharmaceutical-grade deficiency treatment for decades.

After you take it, your liver converts D2 into an intermediate form called 25-hydroxyvitamin D, and then your kidneys activate it into its fully active hormone form. This active form acts like a key that unlocks receptors throughout your body — in your bones, immune cells, muscles, and brain — influencing hundreds of processes. D2 follows the same activation pathway as D3, though research suggests D2 may raise blood levels somewhat less efficiently and may not last as long between doses.

D2 is available over the counter in standard doses (400–2,000 IU) and by prescription in high-dose 50,000 IU capsules for treating clinical deficiency. Liquid drops are available for infants and those who can't swallow capsules. Most fortified foods (plant-based milks, cereals) that are vegan-friendly use D2.

• Vitamin D deficiency
• Weak or brittle bones (osteoporosis)
• Rickets and osteomalacia
• Low immune function
• Muscle weakness
• Mood and seasonal depression
• Poor calcium absorption
• Chronic fatigue

• Rickets (childhood bone deformity from severe deficiency)
• Osteomalacia (adult soft-bone disease from deficiency)
• Falls and fractures in older adults with low vitamin D
• Seasonal immune dips linked to low winter sun exposure

• UV-exposed mushrooms (maitake, portobello, shiitake) — When mushrooms are placed gill-side up in direct sunlight for 15–30 minutes, they generate significant amounts of D2 through the same UV conversion process used industrially. UV-treated maitake can provide several hundred IU per serving.
• Fortified plant-based milks (soy, oat, almond) — Most vegan-certified fortified milks use D2, not D3, to keep them plant-based. A typical serving provides 100–144 IU, making these a reliable daily source for people avoiding animal products.
• Fortified orange juice (D2-fortified brands) — Some brands fortify with D2 specifically for vegan consumers. Check the label — a standard serving typically provides around 100 IU.
• Fortified breakfast cereals — Many ready-to-eat cereals are fortified with D2 and can contribute 40–100 IU per serving. Pairing with fortified plant milk doubles the benefit.
• Ergosterol-rich dried mushrooms — Dried mushrooms that were sun-dried (rather than shade-dried) retain meaningful D2 content. Traditional sun-drying of mushrooms in Asian cuisines was inadvertently a D2 source long before the vitamin was identified.

• Nausea and vomiting (sign of elevated calcium — usually from excessive doses)
• Loss of appetite and weight loss
• Excessive thirst and frequent urination
• Weakness and fatigue (paradoxically, from too much vitamin D)
• Kidney strain or kidney stones (from prolonged calcium elevation)
• Confusion or disorientation (rare, from severe hypercalcemia)
• Headache (occasional, usually mild and dose-related)

• Recommended daily: For general maintenance: 600–2,000 IU daily for adults. For deficiency correction: 1,500–2,000 IU daily (OTC) or 50,000 IU once weekly by prescription for 8–12 weeks. Upper tolerable limit is generally set at 4,000 IU daily for long-term self-supplementation. Always base higher doses on blood test results (target 25-OH vitamin D: 30–60 ng/mL).
• Best time of day: Morning or midday is preferred. Some evidence suggests taking vitamin D with the largest meal of the day improves absorption and may align better with the body's natural cortisol rhythm.
• With or without food: Always take with food — vitamin D2 is fat-soluble and absorbs significantly better when eaten alongside fat-containing foods like avocado, olive oil, eggs, or nuts. Taking it on an empty stomach reduces absorption noticeably.
• Time to effect: Blood levels begin rising within 1–2 weeks. Deficiency correction typically takes 8–12 weeks. Bone density improvements take 6–12 months of consistent supplementation.
• How long to take it: For those with ongoing deficiency risk (low sun exposure, darker skin, older age, vegan diet), long-term daily supplementation is appropriate with periodic blood monitoring every 6–12 months. High-dose prescription courses (50,000 IU weekly) are typically limited to 8–12 weeks, then transitioned to maintenance doses.