Supplement Timing Chart: When to Take 18 Common Supplements (and What Not to Combine)

Most supplements work whenever you actually remember to take them — but about six rules genuinely change how much you absorb. The short version: take fat-soluble supplements (vitamin D, K2, omega-3, curcumin) with your largest, fattiest meal; take iron alone in the morning on an empty stomach, at least 1 hour away from coffee or tea and 2 hours away from calcium; keep zinc and iron doses apart; take magnesium, glycine, and melatonin in the evening; stop caffeine 8–10 hours before bed; and take creatine literally whenever you like, because only the daily total matters. The chart below covers 18 common supplements, and the sections after it walk through the pairs that genuinely conflict, the pairs that synergize, and which timing rules are actually worth the effort.

Doses shown are typical adult supplement ranges, not recommendations. If you take any prescription medication — especially levothyroxine, antibiotics, anticoagulants, or diabetes drugs — confirm spacing with a pharmacist before copying this chart.

What supplements should you not take together?

Only a handful of combinations measurably hurt each other, and almost all of them involve minerals or stimulants.

• Iron + calcium. Calcium is the only common nutrient shown to inhibit both heme and non-heme iron uptake. In a controlled absorption study, 300 mg of calcium taken with an iron supplement cut iron absorption substantially, and the NIH Office of Dietary Supplements advises taking the two at different times of day. Practical rule: iron in the morning, calcium with dinner, minimum 2 hours apart.
• Iron + coffee or tea. In the classic Morck study, a cup of coffee taken with a meal reduced non-heme iron absorption by about 39%, and tea by about 64%. Crucially, coffee drunk 1 hour before the meal had no effect — so the fix is sequencing, not abstinence: coffee first, iron at least an hour later, or iron first thing and coffee after breakfast.
• Zinc + copper (and zinc + iron). Zinc induces metallothionein in gut cells, which traps copper. Chronic zinc above the 40 mg/day adult upper limit can cause genuine copper deficiency with anemia and neurological symptoms, per the NIH ODS zinc fact sheet. High-dose zinc and iron also compete when taken fasted together — separate doses by about 2 hours, and pair long-term zinc with 1–2 mg copper.
• Fat-soluble vitamins + fiber supplements. Psyllium and other bulk fibers can bind fats and fat-soluble compounds in the gut. The human evidence for meaningful vitamin loss is weak, but since spacing costs nothing, take vitamin D, K2, omega-3, and curcumin at a different meal than your fiber dose.
• Almost everything + levothyroxine. This is the interaction that matters most clinically. In a JAMA study, calcium carbonate taken with levothyroxine reduced T4 absorption and raised TSH; iron and magnesium behave similarly by binding the hormone in the gut. Take levothyroxine alone, fasted, and keep calcium, iron, and magnesium at least 4 hours away.

Which supplements work better together?

• Curcumin + piperine. The strongest synergy on this page. Plain curcumin is barely absorbed; in the Shoba trial, adding 20 mg of piperine increased human curcumin bioavailability by 2,000% — a 20-fold difference. Buy a formulation with piperine (or a phytosome/liposomal form) or accept that most of your dose passes straight through.
• Vitamin D3 + K2. Mechanistically sensible: D3 increases calcium absorption, K2 activates the proteins (matrix Gla protein, osteocalcin) that direct that calcium into bone rather than arteries. Direct outcome evidence for the combo is still modest — take them together because both are fat-soluble and pair naturally with the same meal, not because the synergy is proven.
• Iron + vitamin C. Vitamin C reliably boosts non-heme iron absorption in single-meal studies by reducing ferric to ferrous iron. Recent trials suggest the long-term hemoglobin benefit of adding vitamin C to iron therapy is smaller than the absorption studies imply, so treat it as a cheap, low-risk assist: 100–250 mg vitamin C or a glass of orange juice with your iron dose.
• Caffeine + L-theanine. 100 mg L-theanine alongside 50–100 mg caffeine smooths the jitters while preserving alertness — a well-tolerated cognitive stack with decades of combined use.

What does a good morning and evening stack look like?

Morning, with breakfast (a real meal with fat)

• Vitamin D3 + K2 — fat-soluble pair, one swallow
• Omega-3 — same logic; with your largest meal if that is lunch or dinner instead
• B-complex — with food to avoid nausea
• Creatine — here purely for habit-stacking; any time works
• Caffeine + L-theanine — after breakfast, which conveniently keeps coffee away from any iron

Iron protocol (if you supplement iron)

• Iron + 100–250 mg vitamin C, fasted, before coffee, every other morning. In the Stoffel trials, alternate-day dosing produced higher fractional iron absorption than daily dosing because hepcidin — the hormone that gates iron uptake — stays elevated for about 24 hours after each dose.

Evening, from dinner to bed

• Zinc, calcium (if used), probiotics, berberine — with dinner. See our full guide to berberine dosing and evidence.
• Ashwagandha — after dinner
• Magnesium glycinate — 1–2 hours before bed
• Glycine 3 g and/or melatonin 0.5–1 mg — 30–60 minutes before bed

Log each dose in BioTrakk by text or voice and it will line your supplement times up against your sleep and energy data automatically.

Does supplement timing actually matter?

Honest triage: for most supplements, taking the dose at all beats taking it at the perfect time. Missed doses cost you far more than suboptimal ones. Timing genuinely matters in three tiers:

• Critical: iron (empty stomach, away from coffee, tea, and calcium, alternate days), anything near levothyroxine (4-hour rule), caffeine (8–10 h before bed), and berberine (with meals, spaced from interacting prescriptions). Getting these wrong can halve absorption or disturb sleep and thyroid labs.
• Meaningful but forgiving: fat-solubles with a fat-containing meal — worth doing, since taking vitamin D with the largest meal raised serum 25(OH)D by roughly 50% in one clinical cohort — plus evening magnesium and glycine if sleep is the goal.
• Negligible: creatine, vitamin C, L-theanine, ashwagandha, and most everything else. Consistency is the whole game.

Whether a timing change matters for you is an empirical question. A supplement tracker that timestamps every dose, plus wearable sleep data, lets you run the comparison on your own numbers instead of guessing — BioTrakk's n-of-1 experiments do exactly that, with honest positive/negative/inconclusive verdicts.