Key notes

Absorption and timing

Most of the value in supplementing is lost to timing rather than dose.

Split calcium into doses of 500 mg or less; absorption caps out above that.
Iron, calcium, zinc and magnesium compete for absorption — space them two hours apart.
Take iron with vitamin C, and away from tea, coffee and dairy.
Melatonin works on timing, not dose — take it two to three hours before target bedtime.
Take fat-soluble vitamins and omega-3 with a meal containing fat.
Psyllium slows absorption of everything; separate it from all medication by two hours.
Zinc on an empty stomach commonly causes nausea. Take it with food.

Testing and disclosure

Supplements that distort medical tests, and the things your clinician needs to be told.

Tell your pharmacist everything you take, including the things that feel too harmless to mention.
Several supplements need stopping one to two weeks before surgery — ask your surgeon.
Biotin distorts thyroid and troponin lab results. Stop it two days before blood tests.
Saw palmetto lowers PSA and can mask prostate cancer screening.
Folate supplements can hide B12 deficiency while nerve damage continues underneath.
Creatine raises measured creatinine without indicating kidney damage. Tell your doctor.

Tier shifts

The tier is a default, not a diagnosis. These are the cases where a supplement legitimately moves.

Calcium and vitamin D become ongoing after menopause or with osteoporosis treatment.
Iron becomes ongoing with chronic blood loss, malabsorption, or after bariatric surgery.
Folate is Tier 2 generally, Tier 1 during pregnancy planning and the first trimester.
B12 moves to Tier 1 for vegans, over-60s, and anyone on metformin or long-term acid blockers.
A Tier 3 herb taken indefinitely with no condition to treat has quietly become a Tier 4 habit.
Creatine sits in Tier 4 for most people, but behaves like Tier 2 for vegetarians with genuinely lower baseline stores.

Where the harm is

The failure modes that actually send people to hospital. Worth knowing even if you take none of these.

St John's wort can make hormonal contraception fail. This is not theoretical.
Stacking is the most common way people exceed limits — a multivitamin plus a single nutrient plus fortified food.
Red yeast rice is a statin by another name, with every statin risk attached.
Concentrated green tea extract has caused acute liver failure. Brewed tea has not.
Excess iron is stored, not excreted. Never supplement it without a ferritin test.
Prolonged high-dose B6 causes peripheral neuropathy that can persist after stopping.
Powdered caffeine has killed people through measurement error. Never eyeball the dose.
Fat-soluble vitamins A, D, E and K accumulate. This is where more is genuinely worse.

Where the money goes

Not everything expensive is useless, but these are the reliable ways to overpay.

Magnesium oxide is cheap because it is poorly absorbed — mostly a laxative.
BCAAs are largely redundant if your total protein intake is already adequate.
“Proprietary blend” on a label means you cannot know the doses you are paying for.
Price per gram varies enormously between brands for chemically identical compounds.
Collagen powder is largely a glycine delivery system at several times the price.
Read the active amount on the back, not the total capsule weight on the front.
Greens powders cannot replace vegetables; fibre and volume don't survive the process.