How we compare prices

PerDose answers one question: for a supplement you have already decided to buy, in the form you want, which product is cheapest per dose? We do not recommend what to take and we do not score quality.

What we normalise

Pack prices are not comparable: packs differ in size, servings differ in size, and labels quote amounts in different ways. For every product we work out the total amount of the ingredient in the pack and divide the price by it:

• servings = units in the pack × packs ÷ units per serving
• amount in the pack = servings × amount per serving
• price per standard dose = pack price ÷ amount in the pack × the standard dose
• cost per month = price per standard dose × 30

Every ranked row has a “Show the working” panel with these numbers and the exact words from the retailer’s listing that each one came from.

The standard dose is a comparison unit

Each ingredient has a fixed standard dose (for example 100 mg of magnesium) so that every product is measured on the same scale. It is not a recommendation of how much to take. You can type your own daily amount into any table to see costs for that instead.

How listings are read

Retailer listings are read automatically. That step copies facts out of the listing — form, amount per serving, pack size, serving size — and quotes the words each fact came from. It never sees a price and never calculates anything. A number without a matching quote from the listing is thrown away. All arithmetic is done afterwards by ordinary, tested code using the rules on this page.

How often we get it right

Every rule on this page is tested against a set of real supplement labels before anything is published. On 29 October 2026 the rules priced 561 of 561 of those labels correctly, and the automatic reading of listings got 560 of 561 right. Anything the reading is unsure of is left out: see what is left out. What we got wrong and fixed is listed, by date, on the corrections page.

One product, worked through

“Ginkgo Biloba Tablets 6,000mg” at Simply Supplements, £10.69 on 29 October 2026. Each serving is 2 tablets holding 240 mg of ginkgo biloba (extract weight stated on the label). The name on the tub is not the dose; the amount on the nutrition panel is.

• Servings: 180 ÷ 2 per serving = 90
• In the pack: 90 × 240 mg = 21.6 g
• £10.69 ÷ 21.6 g × 120 mg = 5.94p per 120 mg

So this product costs 6p per 120 mg of ginkgo biloba, about £1.78 a month at one standard dose a day. The same working sits under every row on the site.

What we estimate

Minerals are sold as compounds (magnesium bisglycinate, zinc picolinate). What matters for comparison is the amount of the mineral itself. If a label states it, we use it. If a label gives only the weight of the compound, we estimate the mineral content using the fraction below and mark the figure “estimated”. The fractions are calculated from each compound’s chemical formula and standard atomic weights. Real raw materials often contain water or added buffers, so an estimate can be higher than the true figure — a stated amount always wins.

What we do not compare

Products are ranked only against others of the same form. Magnesium citrate is never ranked against magnesium bisglycinate, creatine monohydrate never against creatine HCl, MK-7 never against MK-4. Products with several active ingredients are not ranked alongside single-ingredient ones. You can switch them into a table with “Include combination products”; they are then marked as combinations, their price counts the one ingredient only, and they are never labelled the cheapest. All products, on the Products page, lists them with what a serving contains and the price of a serving, and finds those holding at least the amounts you pick. Table prices leave out delivery, because it depends on the size of your basket;Build a basket adds each retailer’s delivery charge, and the terms are listed below.

What is left out

If a listing does not let us confirm the amount per serving, what that amount refers to, the pack size or the form, we do not guess, and we do not show the product. A common example: “Magnesium Citrate 200mg” could mean 200 mg of magnesium or 200 mg of magnesium citrate — a six-fold difference. Such a product stays off the site until its label has been read and confirmed, and appears in the tables the day it is. Nothing is ever ranked on a figure that was not stated.

Testing claims and dietary labels

Badges such as “testing claimed on label” or “claimed: vegan” repeat what the retailer’s listing says. We have not verified them.

Prices and updates

Prices were last checked on 29 October 2026. Each table and product page shows the date its prices were checked. The price shown is the retailer’s selling price on that date, including any sitewide promotion running at the time; voucher codes are not applied. Always confirm the price on the retailer’s site.

Prices per dose, and what a dose a day costs over a week, a month and a year, are rounded to the nearest penny: 59p below £1, £1.25 from £1. Products are ranked on the exact figure, which the working under each one gives (8.33p rather than 8p), so two products can both read 8p and still be in the right order.

Price history and typical prices

Every product's price is recorded the first time it is checked (the record starts on 17 September 2026), and again whenever the price or the stock changes, so each product page can chart its price since then and list every change with its date. Nothing is estimated between checks: a line stays flat until a check finds a new price. A past price per dose uses the label as it is read today, so a line moves only when the price did.

All products, on the Products page, sets each product against the typical price of its form: the middle price per dose of the products of that form in stock at UK retailers, once there are 3 or more, compared on the amount of the active ingredient each provides. The price index compares each product with its own previous price, day by day, and moves by the average change, so a product arriving or leaving never moves it.

Sale prices and usual prices

The sale prices page lists prices that have dropped against the same product's own record at the same shop. A shop's crossed-out price, RRP or banner is never used: the only comparison is with the record above, so any price on the page can be checked by hand against the product's price history. These are the rules, as they stand today; they are design choices and not measurements.

Usual price

The middle price (the lower of the two middle prices where the count is even, so it is always a price the product really had) among the prices the product showed on the days it was in stock over the last 30 days. It needs at least 7 such days, and it must be a price the product was on for at least half of them: a product whose price keeps moving about has no usual price.

A fall

The price now is at least 10% and at least 50p below the usual price. The product is in stock, ranked, and from a shop dispatching in the UK, and its price was read today or at most 2 days ago: an older price is never shown as a sale.

Lowest yet

The price now equals the lowest in-stock price ever recorded for the product (the record starts on 17 September 2026) and is at least 5% below its usual price.

Shops compared

Only shops whose prices are read every day. Between two readings the record says only that a price held, so a shop read by hand every week or so has no days of its own to count, and none of its products is called a sale. A product also has to have been read on at least 80% of the days since it first appeared.

Left off

A fall of more than 60% is left off the page: at that size it is more often a change of pack or a mistake in the shop's data than a sale. Products are never left out of their price tables by any of these rules.

A rise

The mirror of a fall: the price now is at least 10% and 50p above the usual price, and no more than 60% above it.

Last this low

The last day, before the price now began, on which the product was in stock at this price or lower. Where there is none, the price now is the lowest on record for it.

Black Friday watch

From 13 November 2026 to 3 December 2026, the usual price comes from the 30 days before 13 November 2026 instead of the last 30 days. A price raised in the run-up and cut back on the day is then not a fall, and the prices raised in the run-up are listed as rises. A product with fewer than 7 days in stock before the watch has no usual price during it.

Shop pages

Each shop's page for a supplement lists that shop's products for it, priced per dose, and says where the shop's cheapest sits among every UK shop's, whether it is the cheapest or is beaten, and by how much. The order is only ever price per dose, and no shop pays to be on a page or to move up one.

Which shops have a page

A page exists where the shop has at least 1 ranked product in stock for the supplement, over every form, from UK dispatch, a flavour counting as a product. Below that a page would say less than the product page it lists. A shop with at least 3 such pages has a page of its own, and shops that are also brands link to their brand page, which stays separate.

Where the shop sits

In the form where the shop sells most, its cheapest product in stock by the tables' default price per dose, its place in that form's table (the place the table shows), and the cheapest product in stock in the same table. The difference is written from the two figures as the page shows them, each in whole pence.

How old the prices are

Each page says when its shop's prices were checked. A shop read by hand is checked less often than one read every day, and its page says so; it is dated by the last change to its prices, not by the day the site was built.

How we grade research on overview pages

Overview pages describe, for each reason people take a supplement, what published meta-analyses, systematic reviews and randomised trials (found in Europe PMC) have broadly found. Each study’s summary is read automatically, recording the exact words relied on, whether people took the supplement by mouth, how many took part and what the study found. That reading never chooses the grade: the grade comes from these fixed rules, applied by ordinary code, and the page words it as one plain sentence on how much research exists and how far it agrees, never as a verdict on the supplement. The studies behind each page are listed at its foot.

Well studied (Strong)

At least 2 meta-analyses or systematic reviews, the largest (or all the trials together) covering at least 1,000 participants, and at least 80 % of the evidence pointing the same way.

Fairly well studied (Moderate)

At least 1 meta-analysis or systematic review, or at least 3 randomised trials, covering at least 300 participants, with at least two-thirds of the evidence pointing the same way.

Early evidence (Limited)

At least two-thirds of the evidence points the same way, but there are too few or too small studies for Moderate.

Studies disagree (Mixed)

The studies point different ways: no finding carries two-thirds of the evidence.

Too few studies to grade (Insufficient)

No meta-analysis or systematic review and fewer than 2 randomised trials, or the studies do not say clearly what they found.

Each meta-analysis or systematic review counts as 3 trials when we weigh which way the evidence points. Studies whose abstract does not say clearly what they found are left out. A grade says how settled a finding is, whichever way it points: consistent reviews that found no clear difference count as well studied. It describes how much research exists, not the supplement.

Health topics and their sources

Each health topic sets out what UK health bodies and professional bodies advise about supplements for it, a line or two per source with a link to read it in full, and prices only the supplements those sources advise. Nothing on those pages is our own view of what a supplement does. A source has to come from one of these publishers, and none of them sells supplements:

• Health bodies: NHS, NHS inform, NICE, GOV.UK, Food Standards Agency, UK Anti-Doping, MedlinePlus, NIH Office of Dietary Supplements, NIH National Center for Complementary and Integrative Health, CDC, World Health Organization.
• Medical royal colleges, specialist societies and medical charities: British Menopause Society, Women's Health Concern, Royal Osteoporosis Society, British Dietetic Association, Royal College of Obstetricians and Gynaecologists, Tommy's, National Association for Premenstrual Syndromes, Arthritis UK, British Heart Foundation, HEART UK, Alzheimer's Society, Alzheimer's Research UK, Mind, Royal College of Psychiatrists, Guts UK, British Society of Gastroenterology, British Association of Dermatologists, Macular Society, RNIB, Royal College of Ophthalmologists, Diabetes UK, Liver UK, British Association of Urological Surgeons, Royal College of Paediatrics and Child Health.
• Reference sites written or reviewed by doctors: Patient, Mayo Clinic, Cleveland Clinic, Harvard Health Publishing.

The data

Every ranked row on the site is in one file: perdose-prices.csv, 3,987 rows as of 29 October 2026, rebuilt with every daily price check. Each row carries the supplement, form, product, brand, retailer, pack, amount per serving, pack price, price per standard dose, cost per month, stock and the date the price was checked. Unverified products are left out, as are combination products and Amazon. It is published under CC BY 4.0: use it for anything, with a credit to PerDose and a link. The price history is a second file under the same licence, perdose-price-history.csv: one row for each first check, price change and stock change of every product on the site.

No paid placement

We earn a commission when you buy through our links. It never affects the order of a table: ranking is purely price per dose, and no retailer or brand can pay for position. See the affiliate disclosure.

Report an error

Labels change and we make mistakes.Every product has a “tell us” link, or email hello@perdose.co.uk.