Corticosteroids increase your risk of infection by suppressing the immune system at multiple levels, from disabling key immune cells to blunting the inflammatory signals your body uses to detect and fight pathogens. Even a single short course can raise infection risk: a large study of over one million children found that one steroid burst was associated with a 2.2-fold increased risk of pneumonia and a 2-fold increased risk of sepsis within the first month.
How Steroids Shut Down Immune Defenses
Your immune system relies on T cells to recognize and destroy infected cells, and on chemical messengers called cytokines to coordinate the attack. Steroids interfere with both. Their primary immunosuppressive effect in T cells works by rapidly shutting down a critical signaling chain: steroids block the production of a growth signal called IL-2 and the receptor that detects it. Without that signal, T cells can’t multiply or stay active, which means your body can’t mount a full-scale response when a pathogen arrives.
Steroids also suppress NF-κB, a master switch inside cells that turns on dozens of genes involved in inflammation and immune defense. One way they do this is by triggering the production of a protein called GILZ, which directly inhibits NF-κB. The result is a broad dampening of inflammation. That’s exactly what makes steroids useful for conditions like asthma, arthritis, and autoimmune disease. But inflammation is also how your body walls off infections, recruits white blood cells, and destroys invaders. Turning it down leaves the door open for pathogens that a healthy immune system would catch early.
Macrophages, the immune cells that patrol tissues and swallow bacteria, are also impaired by steroids. They become less effective at engulfing pathogens and less likely to send out the alarm signals that summon other immune cells. This means infections can take hold and spread before your body even recognizes there’s a problem.
How Quickly the Risk Rises
You don’t need to be on steroids for months to face increased risk. Research shows a statistically significant jump in serious complications within 5 to 30 days after a steroid prescription is filled. That risk then gradually declines over the following two months. So even a short “burst” of steroids, the kind commonly prescribed for a bad asthma flare or an allergic reaction, carries measurable danger during those first few weeks.
In the pediatric study of over one million children who received a single steroid burst, the infection rate ratios within 5 to 30 days were 2.19 for pneumonia and 2.02 for sepsis, compared to baseline. Those numbers dropped during days 31 to 90, but didn’t disappear immediately. The takeaway: your immune system doesn’t bounce back the moment you stop taking steroids. It takes weeks to fully recover.
Higher Doses Mean Higher Risk
Infection risk scales with dosage in a clear, stepwise pattern. A large U.S. collaboration studying patients with rheumatoid arthritis found that even low-dose steroids (under 5 mg of prednisone daily) raised the risk of serious bacterial infections by about 32%. At 5 to 10 mg daily, risk nearly doubled. Above 10 mg daily, the risk roughly tripled.
For specific infections, the numbers are even more striking. Tuberculosis risk was nearly 3 times higher at doses under 15 mg daily and almost 8 times higher above 15 mg. Risk for a dangerous fungal lung infection called Pneumocystis pneumonia jumped 19-fold in patients taking 30 mg or more daily compared to those on lower doses. That’s why doctors often prescribe a preventive antibiotic for patients who stay above roughly 16 to 20 mg of prednisone daily for more than eight weeks.
Hospitalized pneumonia risk follows the same gradient. At 5 mg of prednisone daily, the risk was 1.4 times higher than normal. Above 10 mg, it rose to 2.3 times higher.
Which Infections Become More Likely
Steroids don’t just make you slightly more vulnerable to the common cold. They open the door to “opportunistic” infections, pathogens that a healthy immune system easily keeps in check but that can become life-threatening when defenses are down. The most concerning include:
- Pneumocystis pneumonia: A fungal lung infection rare in healthy people but a serious threat for anyone on prolonged high-dose steroids.
- Oral and esophageal thrush: Candida yeast overgrowth, especially common with inhaled steroids that deposit medication in the mouth and throat.
- Tuberculosis: Steroids can both reactivate latent TB and increase susceptibility to new infection.
- Herpes viruses: Reactivation of herpes simplex and varicella-zoster (shingles) occurs more frequently.
- Bacterial pneumonia: Ordinary respiratory bacteria become more dangerous when immune surveillance is impaired.
- Fungal infections: Histoplasmosis, coccidioidomycosis (valley fever), and cryptococcosis can disseminate through the body in immunosuppressed patients.
The type of infection you’re most vulnerable to depends partly on your dose and duration, and partly on what pathogens you’re exposed to in your environment. Someone on low-dose steroids for joint pain faces a different risk profile than someone on high-dose steroids after an organ transplant.
Inhaled Steroids Carry Risk Too
Inhaled corticosteroids, the kind used daily for asthma and COPD, deliver much smaller doses than oral steroids. But they aren’t risk-free. A study following more than 160,000 COPD patients for up to 18 years found that current use of inhaled steroids was associated with a 69% increase in the rate of serious pneumonia requiring hospitalization or causing death. That’s a substantial bump for a medication many people use every day for years.
The risk is highest in current users and appears to drop after stopping the medication. Local effects matter too: inhaled steroids suppress immune defenses in the airways and mouth, which is why rinsing your mouth after each use helps prevent thrush and may reduce respiratory infection risk.
What About Vaccines on Steroids
Given how aggressively steroids suppress immune function, you might expect them to render vaccines useless. The picture is actually more nuanced. Multiple studies comparing influenza vaccine antibody levels in people taking steroids (both oral and inhaled) versus people not on steroids found no significant difference in post-vaccination immune response in most cases. One study of asthma patients did find a decreased response to a single flu antigen, but only in the high-dose inhaled steroid group.
There is one notable pattern: patients already taking steroids before vaccination tended to produce lower antibody levels than those who started steroids afterward. Continuous steroid users showed smaller gains after vaccination compared to people starting fresh. So timing matters. If you’re on long-term steroids, vaccines still provide benefit, but the response may be somewhat blunted. Live vaccines, however, are generally avoided during significant immunosuppression because the weakened virus in the vaccine could itself cause infection.
Why Masking Symptoms Makes Things Worse
There’s another layer to steroid-related infection risk that goes beyond immune suppression. Because steroids are powerful anti-inflammatory drugs, they can mask the usual signs of infection. Fever, redness, swelling, and pain are all driven by inflammation. When that response is dampened, an infection can progress silently. You might not develop the high fever or localized pain that would normally prompt you to seek care. By the time symptoms become obvious, the infection may be significantly more advanced and harder to treat.
This is particularly dangerous for abdominal infections, wound infections, and pneumonia, where early symptoms are subtle even without steroids suppressing them further. People on steroids who develop even mild, persistent symptoms like a low-grade fever, unusual fatigue, or a cough that won’t resolve should take those signals seriously, since the normal alarm bells may be muted.