Frequent Infections: Drugs That Cause or Treat It

What Counts as "Frequent Infections"?

Clinicians use the Jeffrey Modell Foundation's 10 Warning Signs to identify adults who need formal immune workup. The list includes four or more new ear infections in one year, two or more serious sinus infections in one year, two or more pneumonias in one year, and recurrent deep-skin or organ abscesses. Reaching any two of these thresholds justifies laboratory investigation rather than a wait-and-see approach. The full warning-sign criteria are reviewed in a 2015 immunology practice guideline published in the Annals of Allergy, Asthma and Immunology.

Defining "Serious" vs. "Nuisance" Infections

Not every URI matters equally. Recurrent viral upper respiratory infections in a daycare worker reflect exposure, not immune failure. The infections that prompt concern are those caused by encapsulated bacteria (Streptococcus pneumoniae, Haemophilus influenzae), opportunistic organisms (Pneumocystis jirovecii, Cryptococcus), or organisms at unusual body sites. A 2019 review in the Journal of Allergy and Clinical Immunology outlines pathogen-pattern recognition as the cornerstone of immune deficiency screening.

Age and Baseline Rates

Healthy adults average two to four upper respiratory infections per year. Children average six to eight. Exceeding those baselines by more than 50 percent, or experiencing any single episode of bacterial meningitis or osteomyelitis, warrants investigation regardless of total infection count.

Causes of Frequent Infections

Primary Immune Deficiencies

Common variable immunodeficiency (CVID) is the most prevalent symptomatic primary antibody deficiency in adults, affecting roughly 1 in 25,000 people. A 2020 analysis in the Journal of Clinical Immunology found a mean diagnostic delay of 6.1 years from symptom onset to confirmed CVID diagnosis, during which patients averaged 3.2 hospitalizations for infection. Selective IgA deficiency is more common (1 in 300 to 500) but usually milder.

X-linked agammaglobulinemia (XLA) presents in boys under 2 with recurrent bacterial infections after maternal antibody wanes. Severe combined immunodeficiency (SCID) presents in infancy.

Acquired Immune Deficiencies

HIV remains the most globally significant acquired cause. Without treatment, CD4 counts below 200 cells/mm³ define AIDS and correlate with opportunistic infections. The Department of Health and Human Services ART guidelines state that antiretroviral therapy initiated at any CD4 count reduces AIDS-defining illness and improves survival.

Uncontrolled type 2 diabetes drives recurrent skin, urinary, and fungal infections through hyperglycemia-mediated neutrophil dysfunction. A 2015 Diabetes Care analysis of 285,000 patients found that each 1% rise in HbA1c was associated with a 12% increase in infection-related hospitalization risk.

Hematologic malignancies, protein-losing nephropathy, and malnutrition each deplete immunoglobulin levels or impair cellular immunity in distinct ways.

Anatomic and Barrier Defects

Asplenia, bronchiectasis, and urinary tract structural abnormalities predispose to site-specific recurrent infections without any true immune defect.

Drugs That Cause Frequent Infections

This is the section most often omitted from general-audience articles. Drug-induced immunosuppression accounts for a substantial share of recurrent-infection presentations in adults over 40.

Corticosteroids

Systemic corticosteroids are the single most common iatrogenic cause of immune suppression worldwide. Prednisone at doses of 20 mg/day for four or more weeks suppresses both neutrophil migration and T-cell responses. A 2016 meta-analysis in PLOS Medicine (43 trials, N=31,139) found that systemic corticosteroids doubled the risk of serious infection compared with placebo, with the risk scaling with cumulative dose.

Inhaled corticosteroids at high doses (fluticasone 500 mcg/day or equivalent) increase pneumonia risk in COPD patients. The TORCH trial (N=6,112) showed a pneumonia rate of 19.6% in the fluticasone-salmeterol arm vs. 12.3% in placebo over 3 years. TORCH results were published in the New England Journal of Medicine in 2007.

TNF Inhibitors and Other Biologics

TNF inhibitors (adalimumab, etanercept, infliximab) block a cytokine essential for granuloma maintenance, which explains their strong association with reactivation tuberculosis. The FDA black-box warning for TNF inhibitors, reflected in the prescribing information for adalimumab, requires TB screening before initiation.

In rheumatoid arthritis trials, TNF inhibitors roughly double serious infection rates compared with placebo. A 2011 Cochrane review of 63 trials (N=21,788) found an odds ratio of 1.37 (95% CI 1.04 to 1.82) for serious infections with adalimumab vs. Control.

IL-6 inhibitors (tocilizumab) blunt fever responses, which can mask infection severity. JAK inhibitors (tofacitinib, baricitinib) carry class-wide FDA black-box warnings for serious infections including herpes zoster reactivation.

Chemotherapy Agents

Cytotoxic chemotherapy causes predictable nadir neutropenia. Absolute neutrophil counts below 500 cells/mm³ define severe neutropenia, at which point bacterial and fungal infections can be rapidly fatal. ASCO guidelines recommend prophylactic granulocyte colony-stimulating factor (G-CSF) when febrile neutropenia risk exceeds 20%.

Calcineurin Inhibitors and mTOR Inhibitors

Cyclosporine and tacrolimus suppress T-cell activation and are used in transplant recipients and autoimmune disease. They substantially increase risk of opportunistic infections including cytomegalovirus (CMV) and BK virus. Everolimus and sirolimus (mTOR inhibitors) impair T-cell proliferation and raise the risk of Pneumocystis jirovecii pneumonia, which is why trimethoprim-sulfamethoxazole prophylaxis is standard in transplant protocols.

Rituximab and B-Cell Depleting Agents

Rituximab (anti-CD20) depletes B cells for six to nine months after a single infusion and can cause prolonged hypogammaglobulinemia. A 2017 analysis in Blood found that 23% of patients receiving rituximab for non-Hodgkin lymphoma developed secondary hypogammaglobulinemia, of whom 40% experienced recurrent or serious infections.

Proton Pump Inhibitors

PPIs (omeprazole, lansoprazole) are less obviously immunosuppressive but consistently associate with community-acquired pneumonia and Clostridioides difficile colitis. A 2017 JAMA Internal Medicine study of 214,000 patients found a 65% relative increase in C. Difficile risk with high-dose PPI use.

Diagnosing the Cause of Frequent Infections

A three-tier workup efficiently separates primary deficiencies, drug-induced states, and acquired systemic causes.

Tier 1: Office Labs (All Patients)

• Complete blood count with differential (neutropenia, lymphopenia, eosinophilia as clues)
• Serum immunoglobulins: IgG, IgA, IgM
• HIV-1/2 antigen-antibody combination test
• HbA1c (fasting glucose if HbA1c unavailable)
• Comprehensive metabolic panel (protein, albumin, creatinine)

An IgG below 400 mg/dL in an adult with recurrent infections is essentially diagnostic of clinically significant hypogammaglobulinemia and warrants immunology referral. Reference ranges and clinical interpretation are detailed in a 2021 practice parameter from the American Academy of Allergy, Asthma and Immunology.

Tier 2: Targeted Follow-Up

If Tier 1 is abnormal or infections involve unusual organisms, extend to:

• Flow cytometry: CD4/CD8 counts, B-cell and NK-cell enumeration
• Specific antibody titers to pneumococcal serotypes and tetanus (functional antibody testing)
• Complement levels: CH50, C3, C4
• Dihydrorhodamine (DHR) flow cytometry if chronic granulomatous disease is suspected

Tier 3: Genetic Testing

Next-generation sequencing panels for primary immunodeficiency genes are increasingly available and clinically actionable, particularly for pediatric presentations or adults with a suggestive family history.

Treatments for Frequent Infections

Treatment depends entirely on the underlying mechanism. A single category label of "frequent infections" covers at least four distinct therapeutic targets.

Treating the Underlying Cause First

Before any immunoglobulin replacement or prophylactic antibiotic, the physician must address modifiable drivers. Stopping the offending drug, optimizing glycemic control to HbA1c below 7%, or initiating antiretroviral therapy for HIV often resolves the infection pattern within months. The CDC's Advisory Committee on Immunization Practices recommends updated vaccination review at every medical visit for immunocompromised adults.

Immunoglobulin Replacement Therapy

For confirmed primary or secondary hypogammaglobulinemia, immunoglobulin replacement is the standard of care. Both intravenous immunoglobulin (IVIG) and subcutaneous immunoglobulin (SCIG) are effective. Typical IVIG dosing is 400 to 600 mg/kg every three to four weeks, titrated to trough IgG levels above 500 mg/dL or, in patients with bronchiectasis, above 700 to 800 mg/dL.

A 2017 Cochrane review of IVIG in primary antibody deficiencies concluded that replacement therapy reduces acute serious infections compared with no treatment, with SCIG offering comparable efficacy to IVIG with better tolerability profiles.

SCIG allows home self-administration in weekly doses of 100 to 200 mg/kg, improving adherence. Hizentra and Cuvitru are FDA-approved SCIG formulations.

Prophylactic Antimicrobials

Trimethoprim-sulfamethoxazole (TMP-SMX) one single-strength tablet daily prevents Pneumocystis jirovecii pneumonia in patients with CD4 below 200, solid-organ transplant recipients, and those on equivalent prednisone doses above 20 mg/day for more than one month. The HHS opportunistic infection guidelines assign this a Grade AI recommendation.

Acyclovir 400 mg twice daily prevents herpes zoster reactivation in patients starting JAK inhibitors or high-dose corticosteroids. Fluconazole prophylaxis is used in recipients of allogeneic hematopoietic stem cell transplants during the engraftment period.

G-CSF for Neutropenia

Filgrastim (G-CSF) shortens the duration of chemotherapy-induced neutropenia. A 1991 NEJM trial (N=199) showed filgrastim reduced febrile neutropenia incidence from 77% to 40% in small-cell lung cancer patients receiving myelosuppressive chemotherapy. Pegfilgrastim provides the same benefit with once-per-cycle dosing.

Vaccination as Prevention

Vaccines are the most cost-effective intervention in immunocompromised patients who retain sufficient antibody production capacity. Key vaccines include:

• PCV15 or PCV20 (pneumococcal conjugate), followed by PPSV23 at least 8 weeks later for high-risk adults
• Annual inactivated influenza vaccine (live-attenuated influenza is contraindicated in severely immunocompromised patients)
• Recombinant zoster vaccine (Shingrix) for adults over 50 or immunocompromised adults over 19
• Hib vaccine for asplenic patients

The ACIP 2023 adult immunization schedule provides specific catch-up and high-risk dosing recommendations.

Bone Marrow Transplant for Severe Primary Deficiencies

SCID is fatal without hematopoietic stem cell transplant (HSCT). Outcomes are best when HSCT is performed before 3.5 months of age, before any serious infection occurs. Newborn screening for SCID using T-cell receptor excision circle (TREC) assays is now standard in all 50 US states.

Managing Drug-Induced Immune Suppression Specifically

When a medication is the confirmed cause of recurrent infections, clinicians face a risk-benefit calculation rather than a straightforward discontinuation order.

Steroid-Sparing Strategies

For autoimmune conditions driven by corticosteroids, steroid-sparing agents reduce infection risk by allowing dose reduction. Methotrexate in rheumatoid arthritis, azathioprine in inflammatory bowel disease, and mycophenolate in lupus nephritis are examples. The goal is reaching a prednisone equivalent of 7.5 mg/day or less, the threshold at which infection risk approaches baseline. An analysis published in Arthritis Research and Therapy found that prednisone doses above 7.5 mg/day independently predicted serious infection in RA patients, with an adjusted hazard ratio of 1.68.

Biologic Dose Reduction or Switching

For patients on TNF inhibitors with recurrent infections, dose reduction or switching to a mechanism with a lower infection profile (abatacept, for instance) may reduce risk. Abatacept showed numerically fewer serious infections than TNF inhibitors in head-to-head observational data. Any switch decision requires rheumatology involvement and re-screening for latent TB and hepatitis B.

Prophylaxis During Chemotherapy

Beyond G-CSF, patients receiving regimens associated with more than 3.5% Pneumocystis risk should receive TMP-SMX prophylaxis throughout chemotherapy. Fluconazole prophylaxis is added for regimens expected to cause prolonged neutropenia exceeding 7 days.

When to Worry and When to Refer

Most adults with four or more upper respiratory infections per year do not have a primary immune deficiency. Daycare exposure, cigarette smoking, allergic rhinitis, and anatomic sinus disease account for the majority. Referral to an allergist-immunologist is appropriate when:

• Any Tier 1 laboratory is abnormal
• Infections involve unusual pathogens or unusual body sites
• A family member has a diagnosed primary immunodeficiency
• Infections continue despite addressing all modifiable factors
• The patient is starting a potent immunosuppressive drug and baseline IgG is low

The American Academy of Allergy, Asthma and Immunology's 2021 practice parameter recommends referral after two serious infections within 12 months, or after any episode of an opportunistic infection.

"The biggest mistake clinicians make is attributing recurrent pneumonia to bad luck rather than asking whether a drug on the patient's medication list is quietly dismantling their immune defenses," according to commentary in the 2020 CVID consensus guidelines. Early medication review shortens the diagnostic pathway by years.