A low white blood cell count, called leukopenia, happens when your blood contains fewer than 4,500 white blood cells per microliter. The normal range is 4,500 to 11,000. White blood cells are your immune system’s front line, so when levels drop, your body has fewer defenders against infection. The causes range from temporary viral infections to chronic conditions affecting the bone marrow.
Infections That Suppress White Blood Cells
This sounds counterintuitive: the very infections your white blood cells are supposed to fight can actually lower their numbers. Viral infections are one of the most common causes of a temporarily low count. Influenza, hepatitis, HIV, Epstein-Barr virus, measles, chickenpox, parvovirus, and cytomegalovirus can all trigger leukopenia. In most of these cases, the drop is short-lived and your count recovers as the infection clears.
HIV is an exception. Because it directly attacks a key type of white blood cell (CD4 T cells), it can cause a sustained, progressive decline without treatment. Hepatitis viruses can also produce longer-lasting effects, particularly when the infection becomes chronic.
Medications Are a Major Cause
Chemotherapy is the most well-known drug-related cause. These medications suppress the bone marrow cells responsible for producing new white blood cells, and the drop is often expected and monitored throughout treatment.
But chemotherapy is far from the only culprit. A surprisingly long list of everyday medications can lower your white blood cell count. The drugs most frequently linked to this effect include:
- Thyroid medications like methimazole, carbimazole, and propylthiouracil
- Antibiotics like amoxicillin, sulfamethoxazole/trimethoprim, vancomycin, and metronidazole
- Psychiatric medications like clozapine (an antipsychotic) and venlafaxine (an antidepressant)
- Seizure medications like carbamazepine and phenytoin
- Anti-inflammatory drugs like ibuprofen, indomethacin, and diclofenac
- Immune-suppressing drugs like tacrolimus and sulfasalazine
Clozapine and thyroid medications carry particularly high risk, which is why patients on clozapine undergo regular blood monitoring. In most drug-related cases, the count recovers after stopping or switching the medication. If you’ve recently started a new drug and your blood work shows a low count, that connection is worth discussing with your provider.
Autoimmune Conditions
In autoimmune diseases, the immune system mistakenly targets the body’s own cells. Sometimes that includes white blood cells themselves. Lupus and rheumatoid arthritis are the autoimmune conditions most commonly linked to low white blood cell counts. In these cases, the body produces antibodies that destroy its own white blood cells or interfere with their production in the bone marrow. This type of drop tends to be chronic and fluctuates with disease activity.
Bone Marrow Disorders
Your bone marrow is the factory where white blood cells are made. When that factory malfunctions, production drops. Several conditions can cause this.
In aplastic anemia, the bone marrow stops producing enough blood cells of all types, not just white blood cells. If your red blood cells and platelets are also low alongside your white blood cells, this pattern of multiple low counts points toward a bone marrow problem rather than a single-cause issue.
Leukemia affects white blood cells more directly. The bone marrow produces abnormal blood cells that multiply rapidly and eventually crowd out healthy white blood cells, leaving you with lower-than-normal levels of functional ones. Multiple myeloma, another blood cancer, also disrupts normal bone marrow function and can cause leukopenia.
Nutritional Deficiencies
Your bone marrow needs specific raw materials to build white blood cells. Vitamin B12 is essential for cell division and the maturation of blood cells. A B12 deficiency not only causes anemia but also reduces the production of lymphocytes, the white blood cells involved in fighting infections and coordinating immune responses. Correcting a B12 deficiency has been shown to restore lymphocyte counts and normalize the balance between different immune cell types.
Folate deficiency produces a similar effect, since folate and B12 work together in cell division. Copper deficiency is a less recognized but well-documented cause, particularly in people who’ve had gastric surgery or take excessive zinc supplements (zinc and copper compete for absorption).
Benign Ethnic Neutropenia
Some people have a naturally lower white blood cell count that carries no health risk at all. Benign ethnic neutropenia is one of the most common causes of chronically low counts in people of African, Middle Eastern, and West Indian descent. Their neutrophil counts (the most abundant type of white blood cell) sit below the standard reference range, but this is a normal genetic variant, not a sign of disease.
The trait is linked to a specific gene variant in the DARC gene on chromosome 1. Research shows that people with this variant have normal bone marrow production and normal immune function. Their white blood cells work just as effectively as anyone else’s, and studies have found no increased risk of infection. The “low” count appears to result from white blood cells being distributed differently in the body, with more residing in the spleen and tissues rather than circulating in the bloodstream. This means a blood draw captures fewer of them even though the total number in the body is normal.
This matters because a result flagged as “low” on a lab report may not actually be abnormal for you. If you’re of African or Middle Eastern descent and your count has been consistently low without any history of unusual infections, benign ethnic neutropenia is the most likely explanation.
How Low Counts Are Investigated
A single low reading isn’t always cause for alarm. One of the first things a provider will do is check whether you’ve had previous blood counts showing the same pattern. A count that’s been slightly below range for years tells a very different story than one that dropped suddenly.
The most useful next step is a complete blood count with a differential, which breaks down your total white blood cell count into its subtypes: neutrophils, lymphocytes, monocytes, and others. This breakdown narrows the cause considerably. A drop in neutrophils specifically points toward drug reactions, autoimmune destruction, or ethnic neutropenia. A drop across multiple types suggests a bone marrow problem. If your red blood cells and platelets are also low, that pattern further supports a bone marrow issue.
A peripheral blood smear, where a technician examines your blood cells under a microscope, provides additional clues. Abnormally shaped cells or immature cells can suggest leukemia or a related disorder. In some cases, a bone marrow biopsy is needed to look directly at where the cells are being made.
What a Low Count Feels Like
Leukopenia itself doesn’t produce symptoms you can feel. You won’t notice a sensation of having fewer white blood cells. What you may notice are the consequences: infections that come more frequently, last longer, or are harder to shake. Recurring mouth sores, skin infections, respiratory infections, or fevers without an obvious cause can all be signs that your immune system is understaffed. Many people only discover a low count through routine blood work done for another reason entirely.