Alongside freeze-dried vials, a growing number of UK peptide shops list “pens”, “cartridges” and “pre-filled” products. The format changes how a listing is described, how the price should be compared and what can go wrong in transit. This guide is about reading those listings and questioning the brand before you buy. It does not cover how any device is used, and nothing here is guidance on handling the contents.
The three listing types
Shops use these words loosely, so read the description rather than the product title.
| Listing term | What is usually being sold | What to look for in the description |
|---|---|---|
| Cartridge | A sealed glass cartridge holding a liquid, sold on its own | Stated mg per cartridge, volume in ml, number of cartridges per pack |
| Pen | A reusable pen body, sometimes bundled with one or more cartridges | Whether any compound is included at all, or only the device |
| Pre-filled pen | A device supplied already holding a liquid | Stated total mg, volume, batch number and a fill or expiry date |
The most common confusion is a “pen” listing that turns out to be hardware only, with the compound sold separately. If the description does not state an mg figure, assume none is included and ask.
How strength is described
Vials are normally sold by total milligrams of dry powder, for example “10 mg”. Liquid formats are described in more ways, and brands mix them:
- Total mg per cartridge. The full stated amount of compound in one cartridge. This is the figure you need for comparison.
- Concentration. Shown as mg/ml. Multiply by the stated volume to get total mg. A 3 ml cartridge at 5 mg/ml states 15 mg in total.
- Pack total. Some listings quote the combined amount across several cartridges, for example “30 mg (2 x 15 mg)”.
When a listing gives a concentration but no volume, or a pack total with no per-cartridge figure, the price cannot be compared properly. Ask the brand to state both before you order.
Compatibility claims
Cartridge listings often say they “fit most 3 ml pens” or name a particular pen design. These are the seller’s claims about physical fit, made without any independent testing you can see. Treat them as a description, not a promise, and ask the brand what the cartridge has actually been matched against. If a bundle depends on a pen and cartridge working together, check what the brand’s returns and claims process says about a mismatch, and remember that our escrow inspection window covers what arrives against what was listed.
Comparing price per mg with vials
Because pre-filled formats include the container, the liquid and often a device, they nearly always cost more per mg than a vial of the same compound. The calculation is the same as for vials: price divided by total stated mg.
| Example listing | Price | Total stated mg | Price per mg |
|---|---|---|---|
| Vial, 10 mg | £45 | 10 | £4.50 |
| Cartridge, 3 ml at 5 mg/ml | £90 | 15 | £6.00 |
| Pre-filled pen, 2 x 10 mg pack | £130 | 20 | £6.50 |
These figures are illustrations, not real listings. Our compound comparison pages show the actual price per mg for each format side by side, using each brand’s stated amount. If a pen body is included, separate its cost where the brand sells it individually, or the comparison will flatter the bundle.
What to check on the label
A liquid product has more to go wrong than a dry vial, so the label carries more weight. Before the inspection window closes, compare what arrived with the listing:
- Compound name exactly as listed.
- Total mg and volume, and whether they match the listing.
- Batch or lot number, printed on the cartridge or pen, not only on the outer box.
- Fill date or expiry date.
- Brand name and a UK contact address.
- Seal condition: no cracks, leaks, cloudy patches or loose caps.
If anything does not match, open a claim while the funds are still held. The steps are on how escrow works.
What to check on the certificate
A certificate for a liquid product should relate to the liquid as filled, not only to the raw powder it was made from. A COA for the powder says nothing about what happened during filling. Look for:
- A batch number that matches the cartridge or pen.
- A measured concentration or total amount, compared with the label claim.
- The testing lab named, with the brand named as the client.
- A test date after the fill date.
Some brands only publish a certificate for the starting powder. That is useful context but it is not the same document. Our COA guide explains each section, and testing 101 covers which methods measure what.
Storage and transit questions to ask the brand
Liquids can be more sensitive to temperature than freeze-dried powder, and a parcel can sit in a van or sorting office for longer than expected. Before ordering, ask:
- What storage temperature does the brand state for this product, and does the label say the same?
- How is it packed for posting: insulated packaging, cold packs, or neither?
- Which delivery service is used, and on which days does the brand post? Posting late in the week can mean a weekend in transit.
- Does the brand have stability data for the liquid at the stated temperature, or is the expiry date an estimate?
- What happens if the parcel is delayed in a heatwave?
Brand dispatch details are collected on our UK delivery page. A brand that cannot answer these questions in writing is telling you something.
Common questions
What is the difference between a peptide pen and a cartridge?
On most UK shops a cartridge is the sealed container of liquid, and a pen is the device it sits in. Some pens are sold empty, so check whether any compound is included.
Are pre-filled pens more expensive than vials?
Usually, per mg. Work out price divided by total stated mg for each format and compare like for like.
How do I work out the total mg in a cartridge?
Multiply the stated concentration in mg/ml by the stated volume in ml. If either figure is missing, ask the brand.
Does a powder certificate cover a pre-filled product?
Not fully. It describes the powder before filling. Look for a certificate tied to the filled batch.
Do UK brands post cartridges with cold packs?
Some do and some do not. Ask the brand how it packs liquid products before you order.