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A butterfly needle, more formally a winged infusion set, is a short, thin needle attached to flexible plastic “wings” and a short length of connecting tubing. The wings give the person inserting it a stable, easy-to-grip surface, and the short needle shaft is designed for shallow, controlled insertion. It’s used mainly for two things: drawing blood (venipuncture) and giving short-term intravenous (IV) access — a single infusion, an injection, or a brief course of fluids — rather than access meant to stay in place for days.
Where to source this: CASRAI’s sister medical-supply business, LAC, stocks winged infusion sets across common gauge sizes for both bulk facility ordering and smaller individual orders. Browse LAC’s butterfly infusion sets category to see current stock.
What Is a Butterfly Needle Also Called? Butterfly Cannula, Scalp Vein Set and Winged Infusion Set
A butterfly needle is a winged infusion set: a thin steel needle with two flexible wings and a length of clear tubing ending in a Luer connector. It is also called a scalp vein set, and people often search for it as a “butterfly cannula” or “butterfly catheter.” Strictly, though, no flexible cannula or catheter stays in the vein: the steel needle itself is the access device, which is why it is used for blood draws and brief infusions rather than days of access.
Parts and how they work
- Needle: a short, thin steel needle that allows a shallow-angle insertion into superficial veins such as those in the hand, wrist or scalp.
- Wings: flexible plastic grips that are held (and often folded together) while inserting, then flattened and taped to the skin.
- Tubing: transparent tubing, commonly about 20-35 cm, which tolerates patient movement and shows a “flash” of blood when the needle enters the vein.
- Connector: usually a female Luer fitting that attaches to a syringe, a vacuum-tube holder or an infusion line.
Newer sets add a slide-and-lock shield that covers the needle after use to reduce needlestick injuries. These descriptions follow the overview of the winged infusion set.
Common gauges at a glance
| Gauge | Hub colour (ISO 6009) | Typical use |
|---|---|---|
| 21G | Green | Routine adult blood draws; among the most popular sizes |
| 23G | Deep blue | Smaller or more fragile veins; also among the most popular sizes |
| 25G | Orange | Very small or delicate veins, including pediatric patients |
Winged sets exist across a wider range (roughly 18G to 27G). Many phlebotomists avoid 25G and 27G sets because they believe a narrow bore causes hemolysis, but laboratory work suggests hemolysis does not increase with a smaller bore, and one clinical trial found similar hemolysis and coagulation results for 21G, 23G and 25G sets (see the winged infusion set overview). For millimetre diameters and length charts, see CASRAI’s blood draw needle size guide.
Butterfly needle for blood draws, injection and IV
For a blood draw, the tubing connects to a vacuum-tube holder, and the tubes are then filled in the correct sequence; CASRAI’s order of draw guide explains that sequence. For injection or a brief IV infusion, the tubing connects to a syringe or infusion line, and the set is removed once the dose or short course is complete. For an ongoing infusion, an IV catheter is the right device instead. Facilities building a full draw setup can compare components in CASRAI’s phlebotomy kit guide.
What Problem a Butterfly Needle Solves
Straight needles work well on visible, well-anchored veins. They don’t work as well on veins that are small, close to the skin surface, easily rolled, or otherwise fragile — the kind commonly encountered in pediatric patients, older adults, oncology patients with veins altered by prior treatment, and anyone whose veins are simply harder to access that day. A butterfly needle’s shorter, thinner needle and its wing-grip design let the person drawing blood or starting an infusion approach at a shallower angle and with finer control than a longer straight needle or catheter typically allows. That control is the entire reason the device exists: it doesn’t do anything a standard needle can’t do in principle, it just makes difficult sticks meaningfully easier and generally less painful for the patient.
Butterfly sets are most often used by phlebotomists, nurses, and other clinical staff for: routine and difficult blood draws, pediatric and geriatric venipuncture, single-dose IV medication or short infusions, and situations where only brief vascular access is needed. They are not the standard choice for infusions that need to stay in place for multiple days — that’s the job of an IV catheter.
How a Butterfly Needle Differs From a Standard IV Catheter
The two devices are easy to confuse because both go into a vein, but they’re built for different jobs. A butterfly needle is a rigid steel needle for the whole time it’s in the vein — it isn’t meant to stay inserted for more than a short procedure, because a steel needle sitting in a moving patient risks damaging the vein. A standard IV catheter (an over-the-needle catheter, most often called a “peripheral IV” or “PIV”) uses a steel needle only to guide a soft, flexible plastic tube into the vein; the needle is then withdrawn and only the flexible catheter is left in place, which is why an IV catheter can safely stay in for continuous fluids, multiple medication doses, or several days of access.
In short: reach for a butterfly needle when the need is a single blood draw or a brief, one-time infusion, especially on a small or difficult vein. Reach for an IV catheter when the need is ongoing access. For a closer look at gauge sizing and how facilities choose between catheter options, see CASRAI’s IV catheter gauge selection guide.
Choosing Between a Butterfly Needle and a Straight Needle
Butterfly needles aren’t automatically the better choice even for blood draws — the two device types trade off differently on draw speed, sample quality, and vein suitability, and defaulting to one or the other regardless of the vein in front of you is a common but avoidable mistake. CASRAI has a dedicated selection guide that works through that decision in more depth: Butterfly Needle vs Straight Needle Venipuncture: Selection Guide, including gauge ranges and where each device tends to perform better.
Gauge (needle diameter) is one of the main variables in that decision. Winged infusion sets are commonly available in 21G, 23G, and 25G, with color-coded hubs (per ISO 6009) making gauge identifiable at a glance — green for 21G, deep blue for 23G, orange for 25G. Smaller-gauge (higher-number) needles are generally gentler on fragile veins but can slow the draw and, in some settings, are associated with a higher risk of sample hemolysis, which is one more reason gauge selection isn’t purely about patient comfort.
Where It Fits Alongside Other Blood-Draw Supplies
A butterfly needle is rarely used alone. A tourniquet is typically applied above the draw site first to make the vein easier to locate and access — see CASRAI’s guide to what a tourniquet is for how that step fits into the broader venipuncture workflow — and the site is prepped with antisepsis before the needle goes in. Vacuum tubes or a syringe attach to the tubing to collect the sample, and CLSI GP41 (Collection of Diagnostic Venous Blood Specimens) is the standard clinical-laboratory reference most facilities’ phlebotomy procedures are built around.
Practical Relevance for Procurement and Research Administration
For anyone stocking a lab, clinic, or study site rather than performing the draw, the practical questions are usually about SKU selection rather than technique: which gauges to keep on hand for the patient population served, whether a safety-engineered (retractable or shielded) version is required under a facility’s sharps-safety policy, and how winged sets fit into a broader phlebotomy or specimen-collection supply list. CASRAI’s phlebotomy lab supply checklist covers butterfly needles alongside the rest of a standard draw kit, and is a useful starting point for building or auditing a stocking list.
Frequently Asked Questions
Is a butterfly needle the same thing as an IV?
Not quite. “IV” usually refers to an IV catheter — a flexible plastic tube left in the vein for ongoing access. A butterfly needle can be used for a brief IV infusion, but it’s a rigid needle, not a catheter, and isn’t meant to stay in the vein for an extended period.
What gauge is a butterfly needle?
Most winged infusion sets used for blood draws are 21G, 23G, or 25G, with the exact choice depending on vein size and how much the sample is at risk of hemolysis at a smaller gauge.
Why would a butterfly needle be used instead of a straight needle?
Mainly for veins that are small, fragile, close to the surface, or otherwise hard to access — common in pediatric, geriatric, and oncology patients. See the dedicated selection guide linked above for the full trade-offs.
Does a butterfly needle hurt less than a regular needle?
It’s commonly reported as more comfortable for difficult sticks because of its shorter needle and shallower insertion angle, though comfort also depends on the individual vein and the skill of the person performing the draw.
