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Harvard Commentaries
Harvard Commentaries
Reviewed by the Faculty of Harvard Medical School

Medical Myths Medical Myths

Needles -- Separating Fact from Fiction

May 09, 2014

By Robert H. Shmerling M.D.

Beth Israel Deaconess Medical Center

When you were a kid, maybe you called them "shots." Or vaccinations. Or injections. Regardless of what you called them, you probably didn't look forward to them. And you probably still don't. 

I remember seeing my pediatrician at the grocery store when I was around 5 years old. I immediately covered my upper arms (where he'd given me shots before) and hid behind my mom. She thought it was funny, but I was in no mood for laughter. I recognized this guy and I did not want him coming at me with a giant needle.

Of course, the needles my pediatrician used weren't "giant." Perhaps the syringe — the tube holding the medicine and the plunger attached — looked big. But it's not the size of the syringe that matters, if you're worried about getting a painful injection. It's just the needle that punctures the skin; even then, it may be only the tip of the needle, that is used.

Neither the size of the syringe nor the length of the needle is an accurate predictor of how painful an injection will be. That's a common medical myth. And misconceptions about injections are important to set right: An estimated 10% of adults avoid vaccinations mostly because they don't like getting injections!

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Needles Are Sometimes Necessary

There are only so many ways to get a medicine into a person. The easiest option is by mouth (eating or drinking).  But that won't work for medicines that are destroyed by the digestive process.

Applying medicine through the skin or eye (as with a nicotine patch or antibiotic eye drops) can work for certain drugs. And suppositories can provide medicine through the lining of the rectum.  

When a person can't take medicine one of those ways, a needle of some sort may be the only option.

The three most common ways to inject a medicine are:

  • Subcutaneous (under the skin)
  • Intramuscular (into a muscle)
  • Intravenous (into a vein)

There are other types of injections for special circumstances. For example, medicines can be injected into an artery, the spinal fluid or a joint.

The bottom line is that for most medicines that can't be taken in pill or liquid form and that have to reach the bloodstream in a high concentration, there's usually no way around having an injection of some sort.

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Needles Are Not All the Same

There are dozens — perhaps hundreds — of types of needles, depending on the use. Some are only slightly thicker than a thread. Others are more like sharp-edged cylinders than a needles.

The diameter of a needle is described by its "gauge": a 33-gauge needle is much thinner (and usually causes less pain) than a 14-gauge needle. When you have your blood drawn, the most common needle size used is a 21-gauge but smaller ones are often used for vaccinations. An insulin needle (for a subcutaneous injection of insulin for diabetes) is often a 29-, 30- or 31-gauge. 

The length is also highly variable. Some needles are short, less than half an inch, while others are 2 inches or longer.   

A tiny needle can be used for injecting insulin subcutaneously because the medicine works well injected just underneath the skin. And medicine is easy to inject through a thin needle; thicker medicines or those requiring more rapid, deeper injections need a larger and longer needle.  

Doctors' training may also affect their choice of needles. When our arthritis practice tried to decide the types of needles we needed (for joint, bursa or tendon cortisone injections), our 10 doctors could not agree on a small number of choices, even though we're all doing the same procedures in much the same ways.

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Why Some Shots Hurt More than Others

The amount of discomfort associated with an injection depends on a number of factors, including:

  • The site of the injection – Injections into a sensitive area (such as the hand) tend to hurt more than a part of the body with fewer nerve fibers (such as the upper arm).
  • The type of injection – Muscle injections tend to hurt more than skin injections.
  • The amount of medicine injected – As medicine exits the needle during an injection, the tissues are stretched.  This causes pain.  A larger volume of medicine tends to cause more pain than a smaller volume.
  • The speed of the injection – A rapid injection increases pressure in the tissues and  may cause pain. On the other hand, the longer the needle is under the skin or in a muscle, the more opportunity there is for the needle to move. This can also cause pain. It may be that for infants or children who cannot fully cooperate, a rapid injection is better. But for more cooperative adults, a slower injection is better.
  • The size of the needle – In general, a larger diameter needle will irritate more nerve fibers and cause more tissue damage than a thinner needle; length tends to have less impact.
  • The anatomic structures in the path of the needle – Certain structures have pain sensors, while others do not. The skin surface, the bone lining and some nerves are quite sensitive to pain.
  • The type of medicine – Some medicines irritate the tissues and may "sting."  For example, intravenous potassium (which may be given during a hospital admission if the blood potassium is quite low) can cause burning or stinging as it travels through a vein.
  • Use of an anesthetic – Some doctors apply an anesthetic gel or a cold spray to numb the site of an injection. This can reduce the discomfort.

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You May Want To Lie Down for This

Some people faint during injections, or even at the sight of a needle. 

It's an involuntary reflex (called the vasovagal reaction). Having blood drawn or getting an injection (or even just thinking about it) slows the heart rate, lowers blood pressure and results in loss of consciousness. Other symptoms are clammy skin, nausea (and later, hunger), and "turning white as a ghost."

If you've had this reaction before, know that you're not alone. And it's not your fault. But it would be a good idea to lie down before having an injection.

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Taking the Sting out of Needles for Children

A review of 71 studies (involving more than 8,000 children) published in 2010  identified the best ways to reduce the pain of childhood injections. These include: 

  • Breastfeeding or providing a sweet solution to drink during vaccinations
  • A quick (rather than slow) injection
  • Rubbing the skin near the site of vaccination before and during the injection (for kids age 4 and older)
  • Applying an anesthetic to the skin
  • Distracting the child and encouraging deep breathing before the vaccination
  • Giving the most painful vaccination last

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The Bottom Line

Like so many things in life, the worst part about many injections is the anticipation. My advice? Don't look. Lie down if you're prone to fainting. It'll all be over in a few seconds.

Robert H. Shmerling, M.D., is associate physician at Beth Israel Deaconess Medical Center and associate professor at Harvard Medical School. He has been a practicing rheumatologist for over 20 years at Beth Israel Deaconess Medical Center. He is an active teacher in the Internal Medicine Residency Program, serving as the Robinson Firm Chief. He is also a teacher in the Rheumatology Fellowship Program.


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