Nicole Galan, RN, is a registered nurse and the author of "The Everything Fertility Book."

Michael Menna, DO, is a board-certified, active attending emergency medicine physician at White Plains Hospital in White Plains, New York.

If you are prescribed a drug to take at home, such as fertility drugs or hormones, vitamin B12 shots, insulin, epinephrine, or cancer medications, you will have some important responsibilities.

You will need to learn how to safely inject yourself with your medication. You'll also need to know how to buy the needles and syringes you need. These items are sold separately. They're designed to fit together securely.

It's important to choose the right size of needle and syringe so you get the correct dose of medicine. Having the right size also makes the injection easier and less painful.

Your healthcare provider or a nurse will show you how to give yourself a shot at home, including which syringes and needles to use. However, it helps to be familiar with the basic principles of needle and syringe sizing so you can avoid errors when it's time to replace them.

This article explains how needles and syringes are sized. It also provides some helpful tips for choosing the right size for the injection you need to give.

Syringes are labeled based on how much liquid they can hold. There are two ways to measure them.

Whichever measurement is used, 1 cc is equal to 1 mL.

If you are injecting your medication at home, you need to select a syringe that will hold the dose you've been prescribed.

For example, if you're supposed to give yourself 3 cc of a drug, you would want to use a syringe that holds exactly 3 cc (or just a little more). If you use a syringe that can only hold 2 cc, you would have to inject yourself more than once. That's extra pain for no good reason.

On the other hand, if you use a syringe that holds 15 cc, it will be harder to see the cc markings. You could easily end up giving yourself too little or too much medication.

Needles are labeled differently than syringes. The packaging will have a number, then a "G," and then another number.

For example, a 22 G 1/2 needle has a gauge of 22 and a length of half an inch.

If you need to inject yourself with a small amount of medication, it will usually be less painful to use a thin, high-gauge needle, not a wider, lower-gauge needle.

For larger amounts of medicine, a wider needle with a lower gauge is often a better choice. While it might hurt more, a wide, low-gauge needle will deliver the drug faster than a thin, high-gauge needle.

As far as the needle length, the best choice will depend on a person's size (a small child would need a shorter needle than an adult). Where you're inserting the needle also matters. Some medications can be absorbed just under the skin. Others need to be injected into the muscle.

Subcutaneous injections go into the fatty tissue just below the skin. These shots are fairly shallow. The needle required is small and short—typically one-half to five-eighths of an inch long with a gauge of 25 to 30.

Intramuscular injections go directly into a muscle. Muscle is deeper than skin, so the needle used for these shots must be thicker and longer. Needles with a gauge of 20 or 22 G and a length of 1 or 1.5 inches are usually best.

You must also think about how much body fat the needle has to go through. A thin person might be able to use an inch-long needle. Someone heavier might need a needle that is an inch-and-a-half-long.

If you have to give injections to yourself or someone else at home, understanding needle and syringe sizes can make that job a little easier.

Syringes can hold liquids or solids. You'll see milliliters (mL) marked on the tube for measuring liquids. You'll see cubic centimeters (cc) for measuring solids. Try to choose a syringe that holds the exact dose you need.

Needles are measured a different way. The first number on a needle label is its gauge. That's how thick the needle is. Higher numbers mean thinner needles. The second number on the label is how long the needle is. You'll need a longer needle if the medication is supposed to go into a muscle. You'll also need a longer needle for larger adults.

If you need to give yourself or someone else shots at home, you or a family member will need to learn how to do so safely.

If you have questions, reach out to the healthcare provider who prescribed the medication. Knowing the basics of needle size labeling will help you avoid making errors when you're giving the shot and replacing your supplies.

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