About PerDose: frequently asked questions
On this page
How does PerDose work? Where does the price data come from?
PerDose reads product listings from UK retailers’ own affiliate data feeds, then works out a price per standard dose for each one.
Each label’s text is read automatically into structured numbers (pack size, serving size, amount per serving), and a fixed set of rules in ordinary code does every calculation from there — that reading step never computes a price. Prices refresh automatically once a day. A listing that the extraction cannot read with enough confidence is set aside as unverified rather than guessed at, and never appears in the ranked table. The full method, including the exact formulas, is written out on our methodology page.
Read how we compare, in full.
How is the dose per product worked out? What is the golden label set?
The dose per product is worked out from the amount per serving and the number of servings in the pack, using the formulas on our methodology page — never a guess.
This includes converting compound weight to elemental or active content where a published factor applies; none of this arithmetic happens in the step that reads the label. The golden label set is a fixed collection of real and realistic label texts with known-correct answers, used to test that the extraction step reads labels accurately before any change to the system goes live; we track what share of the set passes as a quality gate. It is a testing tool, not something shown to shoppers, but it is why we can say the extraction is checked rather than simply trusted.
See the formulas themselves with a worked example.
How do I report an error?
Every product page has a “report an error” link that opens a pre-filled email naming the exact product and its id, so you only need to describe what looks wrong; you can also email us directly at any time.
We rely on reports like this to catch mistakes an automated system might miss — a mismatched product, a wrong pack size, or a price that looks out of date. We look at every report we receive, though we cannot promise a fix by any particular time, since some depend on the retailer’s own feed being corrected.
Find the “report an error” link on any product page.
Why are there no ratings or reviews?
We deliberately do not score or rate products, because judging quality is a different job from comparing price, and a rating system is easy to game or bias in ways that are hard for a visitor to see.
Where a label makes a specific, checkable claim — such as independent batch testing — we repeat that claim as a badge, clearly labelled as what the label says rather than something we have verified ourselves. Beyond that, we set aside any product we cannot read with confidence rather than guess, but we do not go further and rank products by anything other than price per dose.
Read what we do and do not do.
Why are there no Amazon prices?
Amazon’s rules only let a site display its prices through Amazon’s own product data service, refreshed daily and shown for at most 24 hours.
So an Amazon price appears only while that access is live, and disappears the moment it is not, rather than being permanently on or off. We built the Amazon side of PerDose to stay strictly within those rules rather than work around them: every matched product still gets an exact link to the same item on Amazon, without inventing or estimating a price for it. This is a deliberate choice to keep the data trustworthy, even though it means Amazon coverage is less complete than our other retailers.
Read the full Amazon section of our disclosure.
Does PerDose recommend supplements?
No: we compare the prices of products people have already decided to buy; we do not suggest what to take, how much, or whether a supplement is a good idea for you. The “standard dose” used throughout the site is a comparison unit for putting products on the same footing, not a recommended amount.
We also make no claim anywhere on the site linking a product to any health outcome — that rule applies to every page, including our learn pages and this FAQ, without exception. If you are deciding whether to take something, that is a conversation for a pharmacist or GP, not a price comparison site.
Who is behind PerDose, and is it independent?
PerDose is a small, independent site. It is not owned by a retailer, a supplement brand, or an investor with a stake in which products look cheapest, and no one can pay for position.
Learn pages are written from the studies they cite, and every one is checked against those sources before it publishes; the price tables are pure arithmetic, computed the same way for every product by ordinary code, with no editorial judgement involved in the ranking. Contact details are on the about page if you want to get in touch directly.
Read more on the about page.