The injection used to increase white blood cells is called G-CSF, short for granulocyte-colony stimulating factor. The most common version is filgrastim, sold under the brand name Neupogen. A longer-acting version called pegfilgrastim (Neulasta) works the same way but requires fewer doses. Both are given as shots under the skin, typically in the thigh, abdomen, or upper arm.
How G-CSF Injections Work
Your body naturally produces small amounts of G-CSF, a protein made by certain immune and connective tissue cells. This protein tells your bone marrow to ramp up production of neutrophils, the most abundant type of white blood cell and your front line of defense against bacterial and fungal infections.
The injectable version does the same thing at a much higher intensity. It binds to receptors on blood-forming cells in the bone marrow and triggers them to multiply and mature into functional neutrophils faster than they normally would. Those new neutrophils are also more effective at attacking pathogens once they enter the bloodstream. Most people see their white blood cell counts start rising within one to two days of the first injection.
Short-Acting vs. Long-Acting Versions
Filgrastim (Neupogen) is the short-acting form. It’s injected once daily, often for several days in a row, until white blood cell counts recover to a safe level. Because it’s given daily, it requires more frequent monitoring and either clinic visits or self-injection at home.
Pegfilgrastim (Neulasta) is a modified version of filgrastim designed to stay active in the body longer. A single injection after each chemotherapy cycle replaces the need for daily shots. It’s often applied through a small adhesive device placed on the skin that delivers the dose automatically about 27 hours after it’s attached. For many patients, this convenience makes it the preferred option during cancer treatment.
Several lower-cost biosimilar versions of both drugs are now available. Filgrastim biosimilars include Zarxio, Nivestym, and Releuko. Pegfilgrastim biosimilars include Fulphila, Udenyca, Ziextenzo, Nyvepria, and Fylnetra. These work identically to the originals and go through rigorous FDA review before approval.
When These Injections Are Prescribed
The most common reason is chemotherapy. Many cancer drugs damage the bone marrow as a side effect, causing white blood cell counts to plummet and leaving patients vulnerable to serious infections. G-CSF injections are typically started 24 to 72 hours after a chemotherapy session to help the marrow bounce back before the next cycle.
But chemotherapy isn’t the only use. These injections are also prescribed for:
- Bone marrow transplants, where patients need rapid white blood cell recovery after the transplant procedure
- Severe chronic neutropenia, a condition where the body consistently fails to produce enough neutrophils on its own
- Stem cell donation, where filgrastim pushes stem cells out of the bone marrow and into the bloodstream so they can be collected through a blood draw
- Radiation exposure, where high doses of radiation damage the bone marrow and G-CSF can improve survival
- HIV-related low counts, where the virus itself or certain medications suppress neutrophil production
- Myelodysplastic syndromes and aplastic anemia, conditions where the bone marrow doesn’t produce healthy blood cells properly
Understanding Neutropenia Severity
Doctors measure white blood cell health using something called the absolute neutrophil count, or ANC. A healthy ANC is above 1,500 cells per microliter of blood. Below that threshold, you’re considered neutropenic, and the risk of infection rises sharply as the number drops:
- Mild neutropenia: ANC between 1,000 and 1,500
- Moderate neutropenia: ANC between 500 and 1,000
- Severe neutropenia: ANC below 500
- Profound neutropenia: ANC below 100
At severe and profound levels, even minor bacteria that your body would normally fight off easily can cause life-threatening infections. This is the range where G-CSF injections become most critical. Your medical team will monitor your ANC through regular blood draws (complete blood counts) to track how your counts are responding and decide when it’s safe to stop injections.
Bone Pain Is the Most Common Side Effect
Because G-CSF forces the bone marrow into overdrive, the most widely reported side effect is bone pain. In clinical trials of breast cancer patients, roughly two-thirds experienced bone pain at some point during treatment. The pain is most common during the first cycle, affecting about 46 to 53 percent of patients, and tends to decrease with subsequent cycles, dropping to around 24 to 26 percent by the fourth round.
The discomfort is usually mild to moderate. It’s most often felt in the lower back, pelvis, and legs, which makes sense since those bones contain the most marrow. Only about 5 to 7 percent of patients experience severe bone pain.
Over-the-counter pain relievers are the standard first approach. A randomized clinical trial found that naproxen (an anti-inflammatory painkiller) was effective at reducing both the severity and duration of G-CSF-related bone pain. Acetaminophen is also commonly used. For the small percentage of patients who don’t get relief from these, antihistamines, stronger pain medications, or a dose reduction may be considered.
What Self-Injection Looks Like
Many patients give themselves G-CSF injections at home after receiving training from a nurse. The process is similar to how people inject insulin: a prefilled syringe is inserted just beneath the skin at a 45- or 90-degree angle, typically rotating between the abdomen and thighs to avoid irritation at one spot. Each injection takes only a few seconds.
For pegfilgrastim, some patients use an on-body device that sticks to the skin (usually on the back of the arm) and delivers the medication automatically on a timed delay. This eliminates the need to return to the clinic the day after chemotherapy for the shot. The device is removed and discarded once it finishes delivering the dose.
Filgrastim needs refrigeration and should be brought to room temperature for about 30 minutes before injection to reduce discomfort. Your care team will tell you exactly how many days to inject based on your blood counts and treatment schedule. Skipping doses or stopping early can allow your counts to drop back into dangerous territory before your marrow has fully recovered.