How the on-body injector for a growth factor shot works

Some chemotherapy regimens need a growth factor dose the day after treatment. Historically that meant coming back to the clinic. The on-body injector solves that by sticking a small automatic device to your skin before you leave, timed to deliver the dose at home.

FDA patient instructions describe how it is applied: directly to the skin using a self-adhesive backing, placed on the left or right side of the abdomen, avoiding a two-inch area around the navel. The back of the upper arm is used only if a caregiver is available to help. The provider chooses skin that is intact and not irritated, avoiding surgical sites, scars, and places where tight clothing could disturb it.

You walk out wearing it. That is the whole idea.

The instructions give two numbers that are worth writing down.

• The device delivers the medicine about 27 hours after your provider places it on your skin.
• Delivery takes around 45 minutes to complete.

Twenty-seven hours means the dose lands roughly a day later, a few hours past the same clock time. If your device went on at four in the afternoon, it will start working late the following evening. Knowing this saves you from being startled when the device begins to hum or click while you are watching television.

Write down the time it was applied. Everything else on this page is measured from that moment.

The device tells you what it is doing.

• Flashing green — it is working normally. Do not remove it.
• Solid green, or no light — delivery is complete and it is safe to remove.
• Flashing red with beeping — there is an error. Call your provider right away.

There is also a fill indicator you can look at. FDA instructions describe it reading empty, with a black line, once the medicine has gone in.

The FDA instructions list things that can interfere with the device while you are wearing it:

• Direct sunlight for more than one hour.
• Hot tubs and saunas.
• Diagnostic imaging, including MRI, CT, and X-ray.
• Radiation treatment.
• Oxygen-rich environments.

They also say to avoid travelling, driving, or operating heavy machinery around the window when the dose is delivered — the hours either side of the 27-hour mark. And not to sleep on the device or put pressure on it.

The imaging point catches people out. If you have a scan booked in the same 24 hours, mention the device when you check in.

Two situations need a phone call rather than a decision on your part.

The first is the device coming off. FDA instructions are explicit: do not reapply it, and call your healthcare provider right away, as you may need a replacement dose. The same applies if the adhesive looks noticeably wet or is dripping, which suggests medicine has leaked rather than gone in.

The second is the red flashing light and beeping. That is the device reporting a fault, and it is not something to interpret at home.

In both cases the concern is not the device itself but the dose. This medicine is given to support your white blood cell count after chemotherapy, so a dose that did not arrive is worth knowing about promptly. Your team can arrange a replacement if you tell them in time.

Practically, most people find it easy. It is small, it is under clothing, and it does its job unattended. The main adjustments are sleeping on the other side, being careful getting dressed, and staying somewhere reasonably settled while the dose is delivered.

If you are unsure at any point whether the device has done its job, look at the light and the fill indicator before you take it off — and if you are still unsure, call before you remove it, not after.

• Call 911 or go to an emergency department if you become short of breath, start breathing fast, or your face, mouth or throat swells and you feel faint after the dose goes in.
• Call your care team right away if you get pain in your left upper stomach area or your left shoulder area. The label says this pain could mean the spleen is enlarged or ruptured. It also tells you to call right away for severe pain or skin discomfort around the device.
• Call your care team right away if the device beeped red, came away from your skin before the full dose, or left adhesive that is noticeably wet or dripping. The label says to call at once in each case, because you may need a replacement dose.