Vitamin D and Common Supplements: What to Take and What to Skip
Supplements are not reviewed by the FDA for effectiveness before they go on sale, and the label tells you almost nothing about absorption. A handful are genuinely worth taking, several are worth taking only in a specific form, and a few interact with prescription medicines. Here is how our pharmacists sort them.
Vitamin D — the one most people in the Upper Midwest actually need
Between roughly October and April at our latitude, sunlight is too weak for the skin to make meaningful vitamin D. Deficiency is common, more so in older adults, people with darker skin, and anyone who covers up or works indoors.
The RDA is 600 IU a day (800 IU over 70), and 1,000–2,000 IU daily is a typical and safe supplemental range. Higher doses should be based on a blood level, not a guess. D3 (cholecalciferol) raises blood levels more effectively than D2.
Vitamin D is fat-soluble — take it with the largest meal of the day, which meaningfully improves absorption. It works alongside calcium, so a combined product is reasonable if your dietary calcium is low.
The forms that matter
Magnesium: the form is the whole story. Magnesium oxide is poorly absorbed and mostly acts as a laxative — useful if constipation is your issue, wasteful if it is not. Magnesium citrate and glycinate absorb far better; glycinate is the gentlest on the gut.
Calcium: the body absorbs at most about 500–600 mg at once, so split larger daily totals. Carbonate is cheapest and needs stomach acid, so take it with food — and switch to citrate if you take a PPI or H2 blocker, since citrate does not need acid. Food sources first; very high supplemental calcium has been questioned for cardiovascular risk.
Iron: take it with vitamin C to improve absorption, and away from calcium, coffee, tea and antacids, which block it. Constipation is near-universal, so pair it with fiber or a stool softener. Do not take iron without a reason — unexplained low iron in an adult needs its cause found, not just replaced.
Vitamin B12: genuinely warranted for vegans and vegetarians, adults over 50 (absorption from food declines), and anyone on long-term metformin or a PPI, both of which reduce B12 absorption. That combination is common and rarely mentioned.
Fish oil: reasonable for triglycerides; the evidence for general heart protection in people already on treatment is weaker than the marketing suggests. Take with food and freeze the capsules if they repeat on you.
Melatonin: timing matters more than dose. 0.5–3 mg taken 1–2 hours before bed works better than a large dose at bedtime, and it is a circadian signal rather than a sedative — it helps shifted sleep schedules more than classic insomnia.
Quality, and what to ask a pharmacist about
Because supplements are regulated as food rather than drugs, potency and purity vary. Look for a USP Verified or NSF mark on the label — those are independent tests that what is in the bottle matches the label.
Bring your supplement list to any medication review. The interactions that come up most: vitamin K and warfarin, St John's wort with a long list of prescriptions, calcium and magnesium blocking thyroid medicine and some antibiotics (separate by four hours), potassium supplements with certain blood pressure medicines, and high-dose fish oil with blood thinners.
Take levothyroxine on an empty stomach, and keep calcium, magnesium and iron at least four hours away from it.
What we stock
- Foster & Thrive Vitamin D3 50 mcg (2000 IU) Rapid Release Softgels — 100 ct — $5.67. The everyday dose for most adults. Take with your largest meal.
- Foster & Thrive Vitamin D3 50 mcg (2000 IU) Tablets — 250 ct — $6.30. Eight months' supply for about $6 — best value in the aisle.
- Foster & Thrive Vitamin D3 25 mcg (1000 IU) Rapid Release Softgels — 100 ct — $3.65. Lower dose if you already get some from a multivitamin.
- Silarx Aqueous Vitamin D Oral Drops — 50 mL — $5.80. Liquid drops for infants and anyone who cannot swallow capsules.
- Foster & Thrive Magnesium 250 mg Tablets — 100 ct — $3.67. General magnesium supplementation.
- Foster & Thrive Magnesium Oxide 400 mg Tablets — 60 ct — $4.11. Poorly absorbed — choose this one if the laxative effect is what you want.
- 21st Century Calcium Citrate + Vitamin D Caplets — 400 ct — $14.48. Citrate form — the right pick if you take a PPI or acid reducer.
- Foster & Thrive Calcium 600 mg Tablets — 60 ct — $2.21. Carbonate — cheapest per dose; take with food.
- Foster & Thrive Iron 65 mg (Ferrous Sulfate 325 mg) Tablets — 100 ct — $5.48. Take with vitamin C, away from calcium and coffee. Expect constipation.
- Foster & Thrive Fish Oil 1000 mg Softgels — 100 ct — $9.24. Take with food; freezing the capsules reduces fishy repeat.
- Foster & Thrive Melatonin 3 mg Tablets — 120 ct — $5.44. Take 1–2 hours before bed — timing matters more than dose.
- Culturelle Digestive Health Daily Probiotic Capsules — 30 ct — $18.58. Lactobacillus rhamnosus GG — among the better-studied strains.
Frequently asked questions
How much vitamin D should I take a day?
The RDA is 600 IU daily for adults and 800 IU over age 70, and a supplemental 1,000 to 2,000 IU a day is a common, safe range — particularly in northern states where winter sunlight is too weak for the skin to produce vitamin D. Doses above that should be guided by a blood level. Choose D3 (cholecalciferol) over D2 and take it with your largest meal, since it is fat-soluble.
What is the best form of magnesium to take?
It depends on your goal. Magnesium oxide is poorly absorbed and acts mainly as a laxative, which is useful only if constipation is the point. Magnesium citrate and magnesium glycinate absorb much better, and glycinate is the gentlest on the stomach for general supplementation.
Who needs a vitamin B12 supplement?
Vegans and vegetarians, adults over 50 whose absorption of B12 from food declines, and anyone taking long-term metformin or a proton pump inhibitor such as omeprazole — both reduce B12 absorption. That last group is large and the issue is often missed, so it is worth raising at your next medication review.
How should I take iron so it actually absorbs?
Take it with a source of vitamin C, and keep it away from calcium supplements, antacids, coffee and tea, all of which block absorption. Expect constipation and plan for fiber or a stool softener alongside it. Do not start iron without a reason — unexplained iron deficiency in an adult needs its cause investigated.
Are supplements regulated by the FDA?
Not the way medicines are. Supplements are regulated as food, so they do not need FDA approval for effectiveness before sale and potency can vary between brands. Look for a USP Verified or NSF mark, which means an independent lab has confirmed the contents match the label.
Do supplements interact with prescription medicines?
Some do, meaningfully. Vitamin K affects warfarin; calcium and magnesium block thyroid medicine and certain antibiotics if taken close together (separate by four hours); potassium supplements interact with several blood pressure medicines; high-dose fish oil adds to blood thinners; and St John's wort interferes with a long list of drugs. Bring your full supplement list to your pharmacist.
This guide is general information, not medical advice. Talk with your pharmacist or provider about your situation.