Coring is the accidental punching out of tiny rubber fragments from a medication vial’s stopper when a needle pierces through it. Those fragments can fall into the liquid medication inside the vial and, if not caught, get drawn into a syringe and injected into a patient. It’s a well-recognized hazard in pharmacy and clinical settings, particularly with injectable medications.
How Coring Happens
Every injectable medication vial is sealed with a rubber stopper. When a needle punctures that stopper to withdraw the drug, the sharp tip can slice out a small core of rubber, much like a hole punch cuts through paper. That rubber fragment may stay lodged inside the needle or drop directly into the vial’s contents. Because these particles are often tiny, they can easily go unnoticed, especially in opaque or colored medications where visual inspection is difficult.
The risk increases with repeated punctures. Multi-dose vials, which are accessed multiple times over their lifespan, accumulate more wear and tear on the rubber stopper with each needle insertion. Over time, the stopper becomes more prone to fragmentation, releasing particles that mix invisibly into the medication.
Why It’s a Serious Concern
Injecting rubber particles into the body can cause vascular blockages, embolism, and organ damage. Even microscopic fragments circulating in the bloodstream can lodge in small blood vessels or trigger inflammatory reactions in surrounding tissue. The risk is highest with intravenous medications, where particles have a direct path into the circulatory system, but it also applies to intramuscular and intra-articular injections.
The FDA classifies rubber stopper fragments as one category of particulate contamination in injectable products. Federal regulations require that every final container of an injectable product undergo inspection to detect visible particles, and any unit found to contain them must be rejected. For clear solutions, inspectors use black and white backdrops under controlled lighting to maximize visibility. Opaque products present an obvious challenge and may require advanced technologies like X-ray spectroscopy to detect contamination.
Factors That Increase the Risk
Several variables determine how likely coring is to occur during any given needle puncture.
Needle size is the biggest factor. Larger-bore needles core rubber stoppers far more often than smaller ones. In a controlled study comparing common needle sizes, 18-gauge needles produced rubber particles 38% of the time, compared to just 8% with 20-gauge needles and 6% with 21-gauge needles. That’s a striking difference, and it held regardless of the angle at which the needle entered the stopper.
Needle sharpness also matters, though not in the direction you might expect. Blunt needles actually cause more coring than sharp beveled ones. A sharp tip can slide through the rubber more cleanly, while a duller edge is more likely to tear and displace material.
Insertion angle compounds the problem. Puncturing at 45 degrees with an 18-gauge needle produced coring in 56% of attempts in one study. The combination of a large bore and an angled entry creates the worst-case scenario for stopper damage.
Other contributing factors include the rubber stopper’s composition, the thickness of the diaphragm, and whether the needle’s bevel is facing up or down during insertion.
How Pharmacists and Clinicians Prevent It
The most effective prevention strategies target the needle itself and how it’s used. Choosing the smallest gauge needle that still allows adequate fluid flow is a straightforward way to reduce risk. When a larger needle is necessary for viscous medications, other precautions become more important.
Proper insertion technique helps. Entering the stopper with the needle bevel facing up and applying steady, straight pressure (rather than angling the needle) reduces the chance of cutting out a rubber core. Some protocols recommend inserting the needle at a 90-degree angle to the stopper surface and avoiding lateral movement once the tip has penetrated.
For situations where coring risk is high, filter needles offer a reliable safeguard. These needles contain a built-in filter (typically 5 micrometers) that traps any rubber particles during aspiration. In testing, filter needles eliminated coring contamination completely, catching every particle before it could enter the syringe. They’re inexpensive and practical enough for routine use.
Another option is using prefilled syringes when available. Because the medication never passes through a rubber stopper, there’s no opportunity for coring to occur. This is especially relevant for intra-articular injections (into joints), where rubber particles could cause local inflammation or interfere with the medication’s effect.
In some clinical settings, practitioners remove the rubber stopper entirely before withdrawing medication, bypassing the risk altogether. This approach works when the entire vial contents will be used at once and sterility can be maintained during the process.
Inspecting for Rubber Particles
Visual inspection remains the primary screening method for particulate contamination in clear injectable solutions. The standard approach involves holding the vial or syringe against alternating black and white backgrounds under bright, controlled lighting. Rubber fragments appear as small dark specks that move when the container is gently swirled.
This works well for transparent solutions but becomes nearly impossible with opaque medications, suspensions, or tinted vials. In manufacturing settings, automated inspection systems and supplemental destructive testing fill this gap. At the point of care, the practical solution is prevention: using filter needles, choosing smaller gauges, and employing proper technique so that particles never enter the medication in the first place.