How Inhaled Corticosteroids Treat Asthma and COPD

Inhaled corticosteroids are the most effective anti-inflammatory medications available for controlling asthma, and they have been a cornerstone of treatment for over fifty years. They work by delivering a small dose of steroid directly to the airway lining, which dials down the chronic inflammation that drives symptoms like wheezing, coughing, and shortness of breath. Because the drug lands where it is needed rather than flooding the whole body, the side-effect profile is far milder than that of oral steroids. Still, “milder” does not mean zero, and the details of how these drugs are designed, delivered, and dosed matter more than most people realize.

How Inhaled Corticosteroids Calm the Airways

Airway inflammation in asthma and related diseases involves dozens of genes being switched on inside immune cells and the cells lining the airways. Inhaled corticosteroids suppress that inflammation mainly by reversing the chemical switches that keep those genes active. In practical terms, the drug molecule enters airway cells, binds to a receptor, and the activated receptor moves into the cell’s nucleus where it recruits an enzyme that effectively turns off the inflammatory genes.¹ The result is less swelling, less mucus production, fewer immune cells flooding into the airway wall, and reduced sensitivity to triggers like allergens and cold air.

Over time, this anti-inflammatory action does more than just relieve symptoms. Evidence suggests that high doses of inhaled corticosteroids can reduce some of the structural damage chronic asthma causes, including thickening of the tissue beneath the airway lining and an overgrowth of blood vessels in the airway wall.² That structural damage, sometimes called airway remodeling, is part of why long-standing uncontrolled asthma can become progressively harder to treat. Getting inflammation under control early is one of the strongest arguments for consistent ICS use.

From Asthma to COPD and the Eosinophil Question

Asthma is the classic reason people are prescribed an inhaled corticosteroid, but these drugs also play a role in chronic obstructive pulmonary disease. In COPD the picture is more complicated, because the inflammation driving the disease responds less predictably to steroids. Clinicians now lean heavily on a blood test, the eosinophil count, to decide which COPD patients are likely to benefit. Eosinophils are a type of white blood cell associated with steroid-responsive inflammation. Higher counts at the start of treatment predict a greater response to ICS.³

A meta-analysis found that COPD patients whose blood eosinophils were at or above a two-percent threshold saw about a 17% reduction in moderate-to-severe flare-ups when treated with ICS compared to those on non-ICS therapy. But the same group also had a roughly doubled risk of pneumonia-related events.⁴ That trade-off, fewer flare-ups but more pneumonia, is central to the ongoing debate about who with COPD should receive these drugs and for how long.

Getting the Drug Where It Needs to Go

One of the biggest practical challenges with inhaled corticosteroids is making sure enough of the drug actually reaches the lungs. Much of an inhaled dose can end up stuck in the mouth and throat, which contributes to local side effects without adding any benefit. Particle size is the main lever. Large particles, those above about 6 micrometers, tend to slam into the upper airway and go no further. Very small particles, below about 2 micrometers, sail past the airways and land deep in the alveoli where they are more likely to be absorbed into the bloodstream. The sweet spot for treating the central and smaller airways is roughly 2 to 6 micrometers.⁵

Inhaler technique also plays a big role. With pressurized metered-dose inhalers, how long and how steadily you breathe in affects total lung deposition. Research using functional respiratory imaging found that a five-second inhalation delivered roughly 42% of the metered dose to the lungs, while a one-second puff delivered only about 23%.⁶ In other words, the same inhaler in the same person can deliver nearly twice as much drug to the lungs simply by breathing in more slowly. Using a spacer, a chamber that attaches to the inhaler, helps solve this for people who struggle with coordination.

Not All ICS Molecules Are the Same

Several inhaled corticosteroid molecules are available, including beclomethasone, budesonide, fluticasone propionate, fluticasone furoate, mometasone, and ciclesonide. They share the same basic mechanism but differ in potency, how long they linger in the lungs, and how much gets absorbed systemically.

Ciclesonide is an interesting outlier because it arrives in the lungs as an inactive “prodrug.” The lung tissue itself converts ciclesonide into its active form, a metabolite called des-CIC.⁷ Any drug that deposits in the mouth and throat stays inactive, which reduces local side effects like hoarseness and oral thrush. Studies of human lung tissue confirm that the active metabolite appears at high concentrations in the peripheral lung within a couple of hours of inhalation.⁸

Budesonide, meanwhile, has an unusual property: it undergoes fatty acid conjugation inside lung cells, essentially getting temporarily stored in a lipid form that prolongs its local action. The absorption characteristics of budesonide from the lung show wide variation between individuals.⁹ These molecular differences between ICS compounds have practical consequences for side-effect profiles, as discussed below.

Combination Inhalers and SMART Therapy

Most people with moderate-to-severe asthma end up on a combination inhaler that pairs an ICS with a long-acting bronchodilator. These combinations are not just for convenience. There is evidence of genuine synergy at the molecular level: the bronchodilator can enhance how the corticosteroid interacts with its receptor, improving the drug’s anti-inflammatory punch.¹⁰

A newer strategy called SMART (Single Maintenance And Reliever Therapy) takes combination therapy a step further. Instead of using an ICS-formoterol combination for daily maintenance and a separate rescue inhaler for sudden symptoms, you use the same ICS-formoterol inhaler for both purposes. The ICS-formoterol combination provides quick relief similar to traditional rescue inhalers like albuterol, while also delivering a small anti-inflammatory dose each time symptoms flare.¹¹ Real-world studies have found that SMART reduces the risk of severe asthma exacerbations by about 60% compared to conventional regimens, and it results in lower total steroid exposure over time.¹² Lower lifetime steroid exposure matters because many ICS side effects are dose- and duration-dependent.

Local Side Effects in the Mouth and Throat

The most common complaints from ICS users are hoarseness and oral thrush, a yeast infection in the mouth. Both stem from the same problem: active drug depositing in the oropharynx rather than the lungs. Hoarseness, technically called dysphonia, may result from the corticosteroid causing changes in the muscles or mucosa of the larynx.¹³ The incidence varies widely depending on the drug, the device, and how carefully the patient follows rinsing instructions.¹⁴

The standard advice is to rinse your mouth and gargle after every dose, and to use a spacer with metered-dose inhalers. Using the lowest effective dose also reduces oropharyngeal drug exposure. Prodrug formulations like ciclesonide largely sidestep this issue because, as noted, the drug is inactive until it reaches lung tissue.

Pneumonia Risk, Especially in COPD

Pneumonia has become the side effect that generates the most anxiety around ICS, particularly in COPD. The risk is real but uneven across different molecules. A large cohort study found that fluticasone propionate users were significantly more likely to develop pneumonia than budesonide users, with an incidence rate roughly 40% higher after adjusting for confounders.¹⁵ That gap widened at higher cumulative doses. A separate study looking at serious (hospitalization-requiring) pneumonia found fluticasone roughly doubled the rate, while budesonide raised it by a much smaller margin.¹⁶ Meta-analyses have confirmed this pattern: pneumonia risk varies by molecule and dose, with fluticasone propionate consistently at the higher end and budesonide and beclomethasone at the lower end.¹⁷

One mechanism behind this elevated pneumonia risk appears to involve the immune system in the lungs. Research in both human samples and mice shows that ICS can suppress cathelicidin, an antimicrobial peptide the lungs use to fight bacteria. When cathelicidin is suppressed, bacteria like Streptococcus pneumoniae proliferate more easily. Restoring cathelicidin in mouse models prevented the ICS-driven bacterial expansion.¹⁸ This finding helps explain why ICS use in COPD, where bacterial infections are already common, carries a pneumonia trade-off that is less pronounced in asthma.

ICS also affect the lung’s microbial community in subtler ways. A study of COPD patients found that fluticasone-based treatment was associated with significant shifts in lung microbiome composition and related immune gene expression in the small airways, effects that were not seen with budesonide-based treatment.¹⁹ The differences between ICS molecules in their microbiome effects may relate to fluticasone’s greater potency and lipophilicity, which keeps it in lung tissue longer.

Eyes, Bones, and Growth in Children

Because a fraction of any inhaled dose enters the bloodstream, long-term or high-dose ICS use can produce some of the systemic side effects associated with oral steroids, though typically at a much milder level.

Cataracts are a concern that has been studied extensively. A systematic review found that daily ICS doses above about 1,000 micrograms of beclomethasone equivalent are linked to a meaningful increase in cataract risk, though systemic steroids increase the risk more.²⁰ One large population-based study found that people who used ICS for more than three years had roughly triple the odds of needing cataract surgery, with the risk higher at high daily doses.²¹ Another study found that the highest lifetime cumulative doses of beclomethasone were associated with a more than fivefold higher prevalence of posterior subcapsular cataracts.²² For most patients on moderate doses, the absolute increase in risk is small, but it is worth periodic eye exams if you have been on ICS for many years.

In children, parents often worry about stunted growth. The evidence here is nuanced. A study in children with severe asthma who took low-dose inhaled budesonide (200 micrograms daily) over several years found normal growth patterns and no suppression of the adrenal glands.²³ However, a study comparing budesonide at 400 micrograms daily with fluticasone at 200 micrograms daily found that the budesonide dose produced more growth suppression, with a measurable decrease in height scores over a year, while fluticasone caused less.²⁴ Adrenal suppression, detectable by sensitive testing, was found in about a quarter of children on moderate ICS doses. A separate study confirmed that while ICS dose and duration were associated with adrenal suppression, the drugs did not have a detectable impact on overall growth or bone health in that cohort.²⁵ The bottom line for parents: low-to-moderate doses of ICS are generally safe for growing children, but the lowest effective dose should always be the target, and regular monitoring by a pediatrician is reasonable.

When Inhaled Corticosteroids Stop Working Well

A frustrating reality is that a subset of patients respond poorly to ICS even at high doses. This “steroid resistance” is seen in some people with severe asthma, in smokers with asthma, and broadly in COPD. The resistance is not all-or-nothing; it is usually a spectrum of reduced responsiveness.

Several overlapping mechanisms can blunt the steroid response. In severe asthma and COPD, the key enzyme that corticosteroids rely on to shut down inflammatory genes can be degraded by oxidative stress.²⁶ Additionally, high levels of inflammatory signaling can physically block the steroid receptor from doing its job inside the cell nucleus.²⁷ Smoking is particularly damaging because it drives oxidative stress in the airways, which directly undermines the molecular pathway ICS depend on.²⁸ For smokers with asthma, quitting is one of the single most effective ways to improve steroid responsiveness.

Using Exhaled Nitric Oxide to Fine-Tune Dosing

One of the persistent challenges with ICS is finding the right dose. Too little leaves inflammation unchecked; too much raises the risk of side effects unnecessarily. Fractional exhaled nitric oxide (FeNO) is a breath test that measures a gas produced by inflamed airways. Higher readings suggest active eosinophilic inflammation that is likely to respond to ICS.

A landmark trial found that adjusting ICS doses based on FeNO measurements allowed patients to use an average daily fluticasone dose of 370 micrograms, compared to 641 micrograms in patients managed with conventional guidelines alone, without sacrificing asthma control.²⁹ A subsequent randomized study confirmed that using FeNO alongside standard guidelines helps clinicians arrive at a more accurate daily ICS dose.³⁰ FeNO testing is not yet routine everywhere, but it is increasingly available in specialist clinics and helps make ICS prescribing less of a guessing game.

Inhaled Corticosteroids During Pregnancy

Many women with asthma worry about continuing their ICS through pregnancy. The concern is understandable but largely unsupported by the data. Uncontrolled asthma during pregnancy carries its own serious risks, including preeclampsia, preterm birth, and low birth weight, and stopping ICS to avoid a theoretical drug risk can leave a real disease unchecked.

A nationwide cohort study found that continuing ICS in the first trimester was not associated with increased risks of adverse maternal or neonatal outcomes, despite the fact that many women discontinue the drugs on their own early in pregnancy.³¹ Guidelines consistently recommend continuing ICS at low-to-moderate doses sufficient to control symptoms and prevent exacerbations throughout pregnancy.³² One review has noted that many of the studies in this area are limited in statistical power, so ongoing data collection matters, but the weight of current evidence supports safety.³³

The Carbon Footprint of Your Inhaler

An issue that has gained attention in recent years is the environmental impact of inhaler devices. Pressurized metered-dose inhalers (pMDIs), the “puffer” type that most people picture, use propellant gases that are potent greenhouse gases. Dry powder inhalers (DPIs), which deliver the drug as a fine powder without propellant, have a dramatically smaller footprint. A cross-sectional analysis in one European region found that pMDIs accounted for roughly 1,000 to 1,100 tonnes of CO₂ equivalent per year, while DPIs accounted for less than 55 tonnes.³⁴

Switching every patient to a DPI is not straightforward, because some people, particularly young children and those with very weak inspiratory flow, cannot generate enough force to use a dry powder device effectively. The pharmaceutical industry is also working on pMDIs that use low-global-warming-potential propellants. Modeling across five European markets suggested that transitioning the entire pMDI fleet to low-impact propellants could deliver the largest emission reductions by 2030.³⁵ For now, if your doctor offers a choice between device types and both suit your clinical needs, the DPI is the greener option.

A Brief History Worth Knowing

Before inhaled corticosteroids existed, the only option for steroid-dependent asthmatics was oral prednisone or prednisolone, drugs that reliably controlled airway inflammation but just as reliably caused weight gain, bone thinning, diabetes, and adrenal shutdown with long-term use. The breakthrough came in 1972, when a study in The Lancet demonstrated that beclomethasone dipropionate delivered by aerosol exerted a strong topical effect on the airways while allowing patients previously dependent on oral prednisolone to taper off, with their natural cortisol levels rising back to normal.³⁶ That single finding, that you could put the steroid directly into the lung and spare the rest of the body, reshaped asthma management permanently. Every ICS prescribed today descends from that idea.