Supplements A–Z

Every vitamin, mineral and supplement we've covered, in one list. Each entry shows the daily need, the upper limit and a link to the full guide. Below: how to read the numbers and convert units.

A

Alpha lipoic acid Energy and heart
Daily need
none set: your body makes it
Doses in studies
600 mg a day by mouth; 300–600 mg by drip
Ashwagandha Sleep and stress
What it is
the plant Withania somnifera; supplements mostly use root extract
Studied for
stress, anxiety, sleep; modest effect
Astaxanthin Skin, hair and joints
What it is
a red carotenoid from the microalga Haematococcus pluvialis
EFSA safe intake
up to 8 mg a day from supplements

B

B vitamins Energy and heart
Who needs a B complex
rarely: a limited or restricted diet
More often needed alone
B12 (vegans, over-50s) or folic acid
Berberine Energy and heart
What it is
a plant compound from barberry, goldenseal and others
Blood sugar
modest drop in type 2 diabetes; inconsistent studies

C

Calcium and vitamin K2 Women's health
Calcium a day
700 mg (NHS) / 1,000–1,200 mg (US)
Upper limit
2,500 mg from all sources (EFSA); US 2,000 mg over 50
Children's vitamins Immunity
Main advice
vitamin D for babies and young children
Breastfed babies
vitamin D from birth
Chlorophyll Gut health
What's in the bottle
usually sodium copper chlorophyllin
Doses in studies
100–300 mg a day in 3 doses
Chromium Energy and heart
Intake (US)
25–35 mcg; 20–30 mcg over 50
EFSA position
essential role not proven, no reference intake
Collagen Skin, hair and joints
Main types
I and III – skin; II – cartilage
Dose studied for skin
2.5–10 g peptides a day for 8–12 weeks
CoQ10 Energy and heart
Daily need
none set: your body makes CoQ10 itself
Doses in studies
100–300 mg a day, sometimes up to 600 mg
Creatine Men's health and sport
Form
creatine monohydrate
Daily dose
3–5 g a day

D

Digestive enzymes Gut health
Authorised claim (UK/EU)
lactase only, 4,500 FCC or more with food
Alpha-galactosidase
less wind after beans in small trials

E

Elderberry Immunity
What it is
berries of the black elder (Sambucus nigra)
Colds
may be shorter and milder; uncertain evidence
Electrolytes Men's health and sport
Workout of about an hour
water and food are usually enough
Long sessions, heat
electrolytes plus carbohydrate; drink to thirst

F

Ferritin Tests and deficiencies
What it shows
your body's iron stores
Iron deficiency
below 15 µg/L (WHO)
Folic acid Women's health
Folate need
200 mcg a day (NHS)
Trying to conceive
400 mcg a day: ideally 3 months before conception to week 12

G

Ginkgo biloba Brain and memory
What it is
extract of the leaves of the Ginkgo biloba tree
Preventing dementia
doesn't work: GEM and GuidAge trials
Glucosamine and chondroitin Skin, hair and joints
Core treatment
exercise and weight management (NICE)
Glucosamine
1,500 mg a day; NICE advises against it
Glutathione Skin, hair and joints
Daily need
none set: your body makes glutathione itself
Doses in studies
250–1,000 mg a day by mouth
Glycine Sleep and stress
What it is
a non-essential amino acid found in protein foods
Dose in sleep studies
3 g before bed

H

Hair vitamins Skin, hair and joints
Normal shedding
50–100 hairs a day
Common causes
illness, stress, weight loss, low iron

I

Inositol Women's health
What it is
a vitamin-like compound the body makes itself
Studied for
mainly PCOS
Iron Women's health
Daily need (UK)
8.7 mg for men, 14.8 mg for women aged 19–49
Women 50+
8.7 mg a day

L

L-arginine Men's health and sport
What it is
an amino acid your body turns into nitric oxide
From food
about 3–5 g a day (Danish data)
L-carnitine Men's health and sport
What it is
a compound that carries fat into mitochondria
Where it comes from
made by the body; found in red meat
L-theanine Sleep and stress
What it is
an amino acid from tea leaves
In a cup of green tea
about 23 mg
Lecithin Brain and memory
What it is
a mix of phospholipids from soya or sunflower
Choline (EFSA)
400 mg a day; 480 mg in pregnancy, 520 mg when breastfeeding
Lion's mane Brain and memory
What it is
the edible mushroom Hericium erinaceus
Studied for
memory, mood, stress; little data
Lutein Skin, hair and joints
Who benefits
intermediate AMD, or late AMD in one eye
Effect
about 25% lower risk of late AMD

M

Maca root Men's health and sport
What it is
an Andean root vegetable related to cabbage
Libido
limited evidence from a few small trials
Magnesium Sleep and stress
Daily need (NHS)
300 mg for men, 270 mg for women
Limit from supplements
250 mg (EFSA); 350 mg (US)
Magnesium B6 Sleep and stress
B6 per tablet
usually 5 or 10 mg
NHS advice on B6
no more than 10 mg a day from supplements
Magnesium citrate Sleep and stress
Magnesium share
about 11–16% of the compound's weight
Typical dose
100–200 mg elemental magnesium a day
Magnesium deficiency Tests and deficiencies
Daily need
270 mg women, 300 mg men (NHS)
Share in serum
less than 1% of body magnesium
Magnesium glycinate Sleep and stress
What it is
magnesium + glycine (bisglycinate)
Magnesium share
about 14% of the compound's weight
Men's supplements Men's health and sport
Almost everyone
vitamin D 10 mcg in autumn and winter
Zinc
9.5 mg a day; no more than 25 mg from supplements
Milk thistle Gut health
What it is
a daisy-family plant; silymarin comes from its fruits
Liver disease
evidence conflicting or too limited
Multivitamins Energy and heart
Preventing disease
not shown (USPSTF 2022; COSMOS)
May help if
limited diet, over 50, vegan, could get pregnant

N

NAC (N-acetylcysteine) Immunity
What it is
acetylcysteine, a medicine ingredient
Status
supplement in the US; prescription-only medicine in the UK
NAD+ and NMN Brain and memory
What they are
NAD+ is a cell coenzyme; NMN and NR are its precursors, forms of vitamin B3
What's proven
they raise blood NAD levels

O

Omega-3 Energy and heart
NHS advice
2 portions of fish a week, 1 of them oily (140 g)
Usual need
250 mg EPA+DHA a day (EFSA)

P

Pregnancy vitamins Women's health
Folic acid
400 mcg a day: ideally 3 months before conception to week 12
Folic acid 5 mg
only if your GP prescribes it
Probiotics Gut health
Check first
the strain: genus, species and code
Typical dose
1–10 billion CFU a day
Propolis Immunity
What it is
a resin mix bees collect from tree buds and sap
Best evidence
lip balm for cold sores: trials of one extract
Psyllium husk Gut health
Fibre target
30 g a day (NHS)
Typical UK intake
about 20 g a day

Q

Quercetin Immunity
Daily need
none set: it isn't a vitamin
Doses in studies
150–1,000 mg a day

R

Reishi Immunity
What it is
the mushroom Ganoderma lucidum (lingzhi)
Sold for
immunity, energy, cholesterol, blood pressure
Resveratrol Energy and heart
Daily need
none set: it isn't a vitamin
EFSA safe intake
up to 150 mg trans-resveratrol a day

S

Seasonal vitamins Immunity
Autumn and winter
vitamin D, 10 micrograms (400 IU) a day
Spring
vitamin D until late March or April
Selenium and iodine Energy and heart
Selenium need
70 mcg (EFSA); 60–75 mcg (UK)
Selenium limit
255 mcg a day (EFSA, 2023), all sources
Skin supplements Skin, hair and joints
Most important
sun protection: SPF 30+ and 4-star UVA
Hyaluronic acid
120 mg a day; two small trials
Sleep supplements Sleep and stress
Melatonin in the UK
prescription-only medicine (MHRA)
Usual NHS prescription
2 mg slow-release, 1–2 hours before bed
Spirulina Energy and heart
What it is
dried Arthrospira cyanobacteria
Protein in 3 g
about 1.7 g

T

Taurine Energy and heart
What it is
an amino acid your body makes itself
From food
about 120–180 mg a day for meat eaters
Turmeric (curcumin) Skin, hair and joints
What it is
the main active compound in turmeric
Joints
small drop in knee osteoarthritis pain; low-quality evidence

V

Vitamin B12 Energy and heart
Daily need
1.5 mcg (NHS); 4 mcg (EFSA)
Most likely to need it
vegans, over-50s, metformin or PPI users
Vitamin C Immunity
Daily need
40 mg (NHS); 95–110 mg (EFSA)
Upper limit
2,000 mg (US); EFSA set none
Vitamin D test Tests and deficiencies
What's measured
25(OH)D (calcidiol)
Unit conversion
ng/mL × 2.5 = nmol/L
Vitamin D3 Immunity
Daily amount (NHS)
10 mcg (400 IU) for adults and children over 1
When
October to March for everyone; all year for some groups
Vitamins A and E Skin, hair and joints
Vitamin A need
600–700 mcg (NHS); 700–900 mcg (US)
Vitamin A upper limit
3,000 mcg preformed vitamin A (EFSA, US)

W

Weight loss Energy and heart
The foundation
a modest calorie deficit, activity and, where needed, treatment from your GP
Best supplement effect
about 1 kg more than placebo over months
Women's supplements Women's health
Almost everyone
vitamin D in autumn and winter
If you could get pregnant
folic acid 400 mcg a day

Z

Zinc Immunity
Daily need (NHS)
9.5 mg for men, 7 mg for women
Upper limit
25 mg (NHS, EFSA); 40 mg (US)

#

5-HTP Sleep and stress
What it is
5-hydroxytryptophan, the raw material for serotonin
Source
seed extract of Griffonia simplicifolia

How to use the A–Z

Supplements are sorted alphabetically. Each entry shows the two key numbers from its guide (usually the daily need and the upper limit) and the section it belongs to. Tap a name to open the full guide: forms, doses, who shouldn’t take it and interactions.

A card index drawer with coloured tabs and a magnifying glass, representing the supplements A–Z — The A–Z gathers every guide in one place

To find what you need quickly:

Tap a letter in the row above the list.
Search by the nutrient, not the brand: “Magnesium”, not a product name.
If you’re starting from a goal rather than a nutrient, open supplements by goal.
For an unfamiliar label term, see the supplement glossary.

Daily need, upper limit and the label figure

One nutrient can have four different numbers, and they’re easy to confuse.

Daily need – what almost all healthy adults need a day from food and supplements together. In the UK this is the reference nutrient intake (RNI) the NHS quotes.
Upper limit (UL) – the most you can take every day without harm. We use EFSA limits and give US limits second.
% NRV – the share of the nutrient reference value printed on UK and EU labels.
% DV – the US equivalent, found on most iHerb products. It’s often higher than the NRV.

Daily need, label figure and upper limit

Four different numbers for one nutrient. Pick a tab

1 mcg = 40 IU

NHS advice, autumn and winter 10 mcg 400 IU
NRV: 100% on a UK/EU label 5 mcg 200 IU
DV: 100% on a US label 20 mcg 800 IU
EFSA upper limit 100 mcg 4,000 IU, food and supplements together

Daily needs: NHS (adults 19–64). NRV: retained EU Regulation 1169/2011, used on UK and EU labels. DV: US FDA. Limits: EFSA (2024). EFSA's magnesium limit covers supplements only.

Look at the Magnesium tab: the daily need is above the upper limit, and that’s not a mistake. EFSA’s 250 mg limit covers supplements only, while the need counts all food. Vitamin D is the opposite: the 100 mcg (4,000 IU) limit includes food and supplements.

Two measuring cylinders: one filled to the green daily-need band, the other above the red upper-limit line — The daily need is a band; the upper limit is a line you don't cross every day

Units table: mg, mcg and IU

Unit	What it is	Label example	How to convert
g	gram	creatine 5 g	1 g = 1,000 mg
mg	milligram	magnesium 200 mg	1 mg = 1,000 mcg
mcg (µg)	microgram	vitamin D 10 mcg, folic acid 400 mcg	1 mcg = 0.001 mg
IU	unit of activity, not weight	vitamin D 400 IU	vitamin D: 1 mcg = 40 IU
mcg RAE	retinol activity equivalent (vitamin A)	900 mcg RAE	1 IU retinol = 0.3 mcg RAE
mg alpha-tocopherol	vitamin E	15 mg	1 IU natural = 0.67 mg; synthetic = 0.45 mg
mcg DFE	dietary folate equivalent	400 mcg DFE	1 mcg DFE = 0.6 mcg folic acid taken with food
CFU	live bacteria in a probiotic	10 billion CFU	1 × 10⁹ CFU = 1 billion
nmol/L and ng/mL	blood test units	25(OH)D 50 nmol/L	1 ng/mL = 2.5 nmol/L

UK labs report vitamin D in nmol/L; US sources often use ng/mL. You can do the most common conversions from the table right here. For beta-carotene, use our separate IU to mcg converter, and the vitamin deficiency tests section explains how to read results.

Convert the units in the table

Pick what to convert and enter the number from the label or your test result

What to convert Vitamin D: IU → mcgRetinol: IU → mcg RAENatural vitamin E: IU → mgSynthetic vitamin E: IU → mg25(OH)D test: ng/mL → nmol/L

Number

25 mcg

The unit is in the line below

1,000 → 25 mcg

Vitamin D: 1 mcg = 40 IU. EFSA's upper limit is 100 mcg (4,000 IU) a day from food and supplements together.

Factors from NIH ODS. Minerals aren't measured in IU.

A big cube made of 1,000 small cells beside a single cell held by tweezers: one milligram equals 1,000 micrograms — A milligram is a thousand times a microgram, so always check the unit

When the general figures don’t apply to you

The needs and limits here are for healthy adults. Talk to your GP or pharmacist about doses if:

You’re pregnant or breastfeeding: folate and iron needs rise, and vitamin A (retinol) supplements should be avoided.
You have kidney or liver disease.
You take regular medicines, especially anticoagulants, levothyroxine or antibiotics.
You’re choosing for a child: doses depend on age and weight.

How to use the A–Z when shopping on iHerb

Open the nutrient’s entry and note the daily need and the upper limit.
On the product page, find the Supplement Facts panel and the amount per serving.
Check the unit: mg, mcg or IU. Convert if needed.
Add the dose to your other supplements and food, and make sure you stay under the limit.

Our guide to supplement labels covers the panel line by line, and the types of magnesium guide shows how elemental amounts work.

Vitamin D on iHerbCheck the label lists both IU and mcg: 400 IU = 10 mcg, 1,000 IU = 25 mcg.

Browse vitamin D

FAQ

How do I convert IU to mcg?

Each vitamin has its own factor. Vitamin D: 1 mcg = 40 IU, so 400 IU = 10 mcg and 1,000 IU = 25 mcg. Vitamin A: 1 IU of retinol = 0.3 mcg RAE. Vitamin E: 1 IU of the natural form = 0.67 mg, of the synthetic form 0.45 mg. Minerals aren’t measured in IU.

What's the difference between % NRV and % DV?

Both show a dose as a share of a reference intake. UK and EU labels use NRV; US labels, which you’ll see on most iHerb products, use DV. 25 mcg of vitamin D is 500% NRV but 125% DV.

Why is the daily need for magnesium higher than its upper limit?

The NHS figure (270–300 mg) counts all food, while EFSA’s 250 mg limit applies only to magnesium from supplements and medicines. Magnesium in nuts, grains and greens doesn’t count towards it.

What's the difference between mcg and mg?

1 mg equals 1,000 mcg. Mixing them up matters: 400 mcg of folic acid is a normal dose, 400 mg is a thousand times more. Labels may write micrograms as mcg or µg.