There’s a few indicators that contribute to high-quality vitamin D supplements, including transparency, purity, freshness, form, and dosage.

Type of Vitamin D

Vitamin D3 (cholecalciferol) rather than D2 (ergocalciferol) is recommended when it comes to vitamin D supplementation. “Vitamin D3 is the most effective and bioavailable form,” says Fazio. “For most people, daily dosing is preferred since it better reflects how the body is naturally exposed to and uses vitamin D.” Fazio, Rigden, and Dieras also suggest a formula with vitamin K2 or magnesium as well. “Vitamin D3 works synergistically with vitamin K2 and magnesium for bone and cardiovascular health,” says Rigden.

Dosage

More isn’t necessarily more when it comes to vitamin D. “Unlike water-soluble vitamins that are excreted when taken in excess, vitamin D is fat soluble and stored in the body, which means both deficiency and excess are possible,” explains Fazio. “The ideal dose depends on your blood levels and personal needs so lab testing is the best way to determine how much you require.”

Experts agree—a blood test is the only reliable way to know if you’re getting enough vitamin D. “This will tell you if you are in need of supplementation or conversely if you are supplementing too much,” says Dieras. Too much, it turns out, can have negative effects on the body. “Vitamin D toxicity can present as constipation, decreased appetite, dehydration, fatigue, confusion, frequent urination, irritability, muscle weakness, and/or vomiting,” adds Dieras.

If you’re not deficient 1,000-2,000 IU a day is sufficient to boost levels but the exact dosage should be talked through with your healthcare provider. In case of severe deficiency, a healthcare professional may recommend a higher dose. And, as always, it’s crucial to work with a professional healthcare provider when adding vitamin D supplements to your routine. “Certain medications (like corticosteroids, weight-loss drugs, and anticonvulsants) can interfere with vitamin D metabolism,” says Ridgen. “And very high doses can interact with calcium and increase risk of kidney stones, so balance is important.”

Form

If your body can’t absorb the supplement you’re taking properly, you’re not benefitting from the nutrients, so form matters. “Different forms of supplements are absorbed differently,” says Dieras. “For example, liposomal vitamins have a higher absorption rate than regular vitamins due to the liposomes in which they are delivered.” Again, for vitamin D, look for an oil-based capsule or liquid form for best absorption.

Quality Assurance

Check for third-party testing and certifications to ensure clean, safe, high-quality supplements. “As with any supplements, third-party testing for purity and potency is essential,” says Dieras. “It ensures that what the manufacturer states is in the supplement is what you are actually receiving.” The supplement industry is widely unregulated so third-party testing is a first line of defense to evaluate what you’re putting in your body. Some certifications to look for? Clean Label Project Certified tests for purity and heavy metals, Non-GMO Project Verified ensures products don’t contain GMOs, NSF (National Sanitation Foundation) tests against contaminants, and USP (United States Pharmacopeia) ensures ingredients produced, procured, and used are of sufficient purity and quality for consumption.