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Does size matter when it comes to needles?

It seems intuitively true that decreasing the size of a needle would make it hurt less, but is it really that simple?

Shots, jabs, pricks—whatever you call it, having a needle inserted into your body is not most people’s idea of a fun afternoon activity. Even if you don’t have a specific needle phobia, injection reactions typically range from neutral at best to quite negative at worst. But what if needles didn’t have to hurt? Or, at least, what if they hurt less? It seems intuitively true that decreasing the size of a needle would make it hurt less, but is it really that simple?

The diameter of a needle (how big it is across) is measured in a unit called a gauge. Because the concept of a gauge pre-dates the 18th century and has been defined in many different, inconsistent ways, it’s worth specifying that needle width is measured in the Birmingham gauge. The bigger the gauge, the smaller the needle. For example, the width of a 7-gauge needle is roughly 4.6 mm (0.18 inch), while the width of a 30-gauge needle is about 0.31 mm (0.012 inch). To give you some context, a typical spaghetti noodle is roughly 14-gauge, and a regular stud earring is about 19-gauge.

There are a few factors that determine what size of needle a practitioner needs to use, including the body size of the patient and the body part being pricked, but a critical factor is the amount of fluid being injected or drawn out of the patient. If you try to inject a large amount of fluid through a very thin needle, it will both take longer and hurt more due to the high pressure.

diagram of the components of a syringe

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For blood collection, which is typically a few millilitres of blood, clinicians use needles of 21-22 gauge. Vaccines are often <1 mL and accordingly use needles that are slightly smaller, around 22-25 gauge. Delivering insulin to diabetic patients requires even less fluid and can use needles as small as 29-31 gauge.

Even with the limitations imposed by volume, there is some wiggle room in the gauge of needle used for a certain procedure. Medical practitioners can often use their own judgment, experience, and clinical guidelines to change the size of needle they use. Much like how artists may favour a certain size brush, some clinicians have personal preferences in the tools of their trade.

Luckily, it is actually relatively simple to study whether decreasing needle diameter decreases pain. Just find some volunteers who are willing to be stabbed for science (or who are already being treated with a needle-involved method), stick them with at least 2 needles of different gauges without telling them which is which, and ask them how much it hurt on a numeric scale. There are dozens of studies that take this form.

Regarding simple injections in the body, this study compared a 30-gauge needle with a 26-gauge one and found no significant difference in the reported pain. As did this study, which compared 27-gauge vs 23-gauge vs 21-gauge. For injecting Botox around patients’ eyes, this study found no difference in pain scores between a 32-gauge and a 30-gauge needle. These are by no means all of the studies on needle size and pain, but they are representative of the scientific literature on this topic. Again and again, trial participants seem to find no significant difference in their pain when comparing needle gauges.

For many people, the anesthetic injection is the worst part of any dentist visit. While it would be lovely to tell you that a quick swap to a thinner needle is all you need to decrease the pain of dental injections, there is a wealth of evidence to the contrary. For anesthetic injections in the mouth, smaller-width needles were not only ineffective at reducing pain; in one study, they actually increased it!

Maybe needle gauge has a significant effect in more niche and uncommon situations? Intravitreal injections involve a needle entering one’s eyeball, which seems intensely unpleasant. Unfortunately, the use of a 30-gauge needle in place of a 27-gauge one did not reduce patients’ pain in a significant way. Similarly, patients receiving lumbosacral epidural injections experienced no difference between 22- and 25-gauge needles.

On the other hand, this study compared 16-gauge needles with either 17- or 20-gauge ones for oocyte retrieval and found a significant reduction in pain scores, not only during the procedure but afterwards, too. For arterial punctures, which involve inserting needles quite deeply into one’s wrist, this study found a large decrease in pain when using a 29-gauge needle instead of a 23-gauge one. When injecting into tumours on the head, this study found a significant decrease in pain scores when a 33-gauge needle was used instead of a 30-gauge one—but only on the face and scalp, not the neck.

Regretfully, reducing the pain of needles is not as simple as reducing their size. For certain procedures, using a thinner, higher gauge needle can make them hurt less, but even then, the effect size is small. In the above-mentioned oocyte retrieval study, the smaller needle reduced patient pain ratings by an average of 18%. For the arterial puncture study, 16%.

Most people do not like getting injections or having their blood drawn, and the anxiety and distress resulting can be made worse by seeing a “big” needle coming your way. But just know while thinner needles might look less intimidating, they are not the universal answer to less painful jabs. By using that bigger needle, your doctor may actually be making it hurt less!


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