A pen delivers medicine through a small needle under the skin. Pens differ in their dose controls, needles, priming steps and storage rules. Ask a health professional to show you how to use your particular device before the first injection. Use a pen and its cartridge for one person only, even if the needle has been changed; sharing can pass on an infection.
Approved prefilled pens
An approved pen comes filled with a particular medicine. Some are single-use devices with one fixed dose. Others contain several doses and have a dose selector. Check the medicine name, strength, expiry date and appearance against your own instructions before each use. A fixed-dose pen has no dose to dial, while a multidose pen may require you to select the prescribed dose in the display. Do not count clicks to estimate a dose.
The steps are specific to the device. For example, the Ozempic multidose pen instructions call for a flow check before the first injection with each new pen. After selecting and injecting the prescribed dose, keep the needle in the skin and slowly count to six after the dose counter reaches zero; then remove the needle. By contrast, the Zepbound single-dose pen has no selector or priming step: its instructions say to press and hold the button for up to ten seconds, until the second click and gray plunger confirm completion. Its needle is built into the pen, which goes into a sharps container after use. Other approved pens can work differently.
Refillable cartridge pens
Some people put reconstituted peptides into a cartridge used in a reusable pen. Using a pen and cartridge this way for a peptide is unapproved use, even if the pen itself is approved for a different medicine. It does not establish that the mixture is sterile, stable, effective or safe. This site does not give instructions for filling pen cartridges. The reconstitution chapter covers vials only. If you are considering this, discuss it with a pharmacist first.
A pen's markings measure the device's delivery scale, not a fixed number of milligrams of every substance. To calculate a dose, you need both the concentration in the cartridge and the volume delivered by the specific pen. A calculator's unit scale must match the pen: a U-100-equivalent unit is 0.01 mL, but a click or a displayed unit on another device may represent a different volume. Do not assume clicks are interchangeable between pens or that an insulin scale is suitable for a different formulation. Hold times differ too: some reusable insulin-pen guidance describes a 10-second hold, but follow the instructions for the exact device. If the scale, concentration or calculation is unclear, stop and check with a pharmacist or clinician before injecting.
Worked example, for arithmetic only: vial 10 mg + 2 mL → 5 mg/mL; a pen dialled in U-100-equivalent units: 10 units = 0.1 mL = 0.5 mg. This assumes the final solution volume is 2 mL and that the pen actually delivers 0.01 mL per displayed unit. It is not a suggested dose. Priming, if the device requires it, uses additional liquid and is not the dose delivered to the body.
Injecting under the skin
Wash and dry your hands. Select a site allowed by the product instructions: the abdomen, at least 5 cm (2 inches) from the navel; the front of the thigh; or, when the product permits, the back of the upper arm, generally with another person helping. Different products allow different sites. Rotate to a different spot for each injection, even if you use the same general area. Avoid skin that is bruised, tender, red, hard, scarred or otherwise damaged. Clean the site as directed and let it dry.
For a pen with a detachable needle, attach a new needle for every injection and perform any device-specific flow check or priming exactly when the instructions say to. Place the pen and inject as shown in the instructions. Keep it in place for the specified time before lifting it away, then remove and discard the needle as directed. A single-use autoinjector already contains its own needle; use a new device for each injection. Do not share or reuse needles. Example site and device instructions.
Storage and missed doses
Check the exact product's instructions for both unused and in-use pens. Many unused pens require refrigeration, while the permitted temperature and time after first use or removal from the refrigerator vary. For example, the Ozempic instructions allow an in-use pen for 56 days under their stated storage conditions; the Zepbound single-dose instructions allow up to 21 days at the stated room-temperature limit. Those times do not transfer to other pens or to self-filled cartridges. Follow the label on whether a pen may return to the refrigerator, protect it from heat and light as directed, and do not use a product that has been frozen when its label says not to. Ask a pharmacist about storage and discard timing for a reconstituted preparation; appearance alone does not prove it is safe.
For a weekly approved medicine, the label may allow the missed dose within a specific window; after that window, it may direct you to skip the dose and resume the usual schedule. For example, Ozempic uses a five-day window, while Zepbound uses four days. For a daily approved medicine, check its label: Rybelsus, for example, says to skip a missed dose and resume the next day. Do not double a dose to catch up. If several doses are missed, a dose may be incomplete, or there is no reliable label for the preparation, contact the prescriber or pharmacist before deciding what to take next.
Disposal
Place used needles and single-use pens with built-in needles straight into an approved sharps container. For a detachable needle, use the removal method in the pen instructions to avoid a needlestick; store a reusable pen without a needle attached. Never put loose needles in household trash or recycling. Seal the sharps container and follow local disposal rules; if an approved container is unavailable, follow local guidance on a heavy-duty, puncture-resistant, lidded alternative. FDA sharps guidance.