Definition: Coring is when a needle cuts a small piece out of a vial's rubber stopper as it goes through. The piece, called a core, can fall into the liquid or stay in the needle and be drawn into the syringe. Vials punctured many times, like multi-dose vials, are more prone to it.
What studies found
In a 2014 test of rubber-topped vials, coring happened in 102 of 250 (40.8%) vials pierced with a blunt plastic safety needle, versus 9 of 215 (4.2%) with a sharp 18-gauge needle (Wani 2014). The authors warn that fragments can be drawn up with the drug and possibly injected.
A 2025 review by a retina specialists' safety committee lists the factors linked to coring: needle size, straight (perpendicular) entry, vials used many times, and thicker stoppers. Stopper material matters too (Ali 2025). The review found no documented harm from coring in eye injections, but called for careful preparation and vial checks.
How to spot it
- A core is stopper material, so it shows up as a small fleck of rubber floating in the liquid or resting at the bottom.
- The bacteriostatic water label says to inspect injectable drugs for particles and discoloration before use, and not to use the liquid unless it is clear and the seal is intact.
- Filter needles are often used to keep particles out of the syringe when drawing from a vial (Ali 2025).
Every puncture adds to the stopper's wear. For how often a multi-dose vial is entered and for how long, see multi-dose vial.
Read more on Dose Track: Vials and inventory in Dose Track · Peptide storage guides
General education drawn from labels and published studies, not personal medical advice. Your prescriber, pharmacist and the label that came with your product are the final word.
Is coring dangerous?+
A fragment can be drawn into the syringe and injected. A 2025 review found no documented harm from coring in eye injections, but it is treated as a safety issue to prevent.
Do blunt needles core more than sharp ones?+
In one study, yes: 40.8% of vials cored with a blunt plastic needle versus 4.2% with a sharp 18-gauge needle.
What should I do if I see a fleck in a vial?+
The bacteriostatic water label says to inspect for particles and not to use the liquid unless it is clear. A fleck of rubber means it is not.