Coring is the accidental removal of a small piece of rubber from a medication vial’s stopper when a needle punctures through it. That tiny fragment can end up inside the syringe and, if undetected, get injected into a patient along with the medication. It happens more often than most people realize: studies have found coring rates ranging from about 4% to over 40% depending on the needle type and technique used.
How Coring Happens
Medication vials are sealed with rubber stoppers. To draw out the liquid inside, a healthcare worker pushes a needle through that stopper. If the needle cuts cleanly, it passes through without incident. But sometimes the needle punches out a small cylinder of rubber, much like a hole punch cuts through paper. That rubber fragment, called a “core,” can travel into the syringe barrel, float in the medication, and potentially be injected into the patient’s bloodstream, muscle, or joint.
Several factors influence whether coring occurs: the material and thickness of the rubber stopper, the size of the vial, the type and gauge of the needle, how many times the stopper has already been punctured, and the angle at which the needle enters. Even the direction the needle’s bevel (the angled opening at the tip) faces plays a role.
Needle Size Makes a Big Difference
Larger needles core more frequently. In a study testing three common needle sizes on multi-dose vials, 18-gauge needles (the widest tested) produced rubber fragments 38% of the time. By comparison, 20-gauge needles cored at 8% and 21-gauge needles at just 6%. The difference was statistically significant, and it makes intuitive sense: a wider bore cuts a bigger circle through the rubber, making it easier to dislodge a fragment.
Blunt needles are far worse than sharp ones. One prospective study found coring in 40.8% of vials pierced with blunt plastic safety needles, compared to only 4.2% with sharp needles. Certain medications were especially prone to the problem. Among vials accessed with blunt needles, coring rates reached 58% for propofol (an anesthetic), 86% for cefazolin (an antibiotic), and 64% for neostigmine. These differences likely reflect variation in stopper material and firmness across manufacturers.
Why It’s a Safety Concern
A rubber fragment floating in a syringe might seem trivial, but the consequences depend on where the medication is going. If injected into a vein, a rubber particle can travel to the lungs or other organs. Foreign material in the bloodstream can block tiny blood vessels, triggering an inflammatory response. The body reacts to the foreign material by forming granulomas, which are small clusters of immune cells that wall off the particle. Over time, this can cause tissue damage at the site where the fragment lodges.
Particles that reach the lungs can obstruct the microvasculature there, potentially causing hemorrhage, fluid buildup, and breathing difficulty. While catastrophic outcomes from a single rubber core are rare, repeated exposures or larger fragments raise the risk. For injections into joints, rubber particles can provoke local inflammation that complicates the treatment the injection was meant to provide.
The Angle Technique That Helps (and Its Limits)
The most widely recommended prevention strategy involves changing how the needle enters the stopper. Instead of pushing straight down at 90 degrees, clinicians are advised to insert the needle at a 45 to 60 degree angle with the bevel facing upward. This approach can reduce coring by roughly 50%, because the angled entry lets the needle slide through the rubber rather than punching out a disc.
There’s an important catch, though. For 18-gauge needles specifically, the angled technique actually backfires. One study found that 18-gauge needles inserted at 45 degrees cored 56% of the time, compared to 20% when inserted straight at 90 degrees. The likely explanation is that the wide bore of the needle, combined with the angled approach, creates a larger cutting surface that shears off more rubber. For smaller needles (20- and 21-gauge), the insertion angle made no significant difference either way.
This means the single most effective prevention step is choosing a smaller needle when possible. Puncturing the stopper only once, rather than re-entering a multi-dose vial repeatedly, also helps reduce the odds.
What Happens When Coring Is Detected
Rubber fragments are sometimes visible to the naked eye as small dark specks floating in the syringe or sitting at the bottom of a drawn-up medication. Current safety guidance from the Institute for Safe Medication Practices calls for a straightforward protocol when coring is spotted:
- Do not inject the medication. If there’s any suspicion of rubber contamination, the dose should be discarded.
- Set aside the vial and syringe. Both should be secured, not thrown away, so they can be examined and reported.
- Contact the pharmacy. The pharmacist can help document the event and coordinate returning the affected vial to the manufacturer.
- File a report. Details like the medication name, manufacturer, lot number, needle type, and gauge should be included. Photos of the visible fragment are useful for tracking patterns.
The challenge is that not all cores are large enough to see. Microscopic rubber particles can pass through visual inspection entirely, which is why prevention through proper technique and needle selection matters more than relying on catching the problem after the fact.
Coring in Other Contexts
Outside of medication preparation, the word “coring” describes any process where a cylindrical plug of material is removed from a larger piece. In dermatology, a punch biopsy works on exactly this principle: a sharp, hollow circular instrument about the size of a pencil eraser is pressed into the skin and rotated to cut out a small round sample roughly 4 millimeters deep, reaching down through all skin layers to the fatty tissue beneath. The “core” of tissue is then examined under a microscope to check for skin cancer or other conditions.
In food preparation, coring refers to removing the central seed-bearing section of fruits like apples or pineapples. In manufacturing and drilling, it describes extracting cylindrical samples from rock, concrete, or other materials for testing. The underlying concept is always the same: a hollow tool cuts and removes a plug from something solid.