An MDI, or metered dose inhaler, is a small handheld device that delivers a precise, pre-measured dose of medication directly into your lungs. It’s the most common type of inhaler used to treat asthma and chronic obstructive pulmonary disease (COPD), and the one most people picture when they think of an inhaler: a small pressurized canister sitting inside a plastic mouthpiece.
How an MDI Is Built
Every MDI has three core parts: a metal canister, a metering valve, and a plastic actuator (the boot-shaped housing you hold and put to your mouth). The canister holds the medication mixed with a liquefied propellant under pressure. The metering valve sits at the bottom of the canister and controls exactly how much medication is released with each press. The actuator is the outer shell that holds everything together and shapes the spray as it exits the mouthpiece.
Many newer MDIs also include a built-in dose counter on the back of the device. The FDA has recommended since 2003 that manufacturers integrate dose counters into new MDI products so patients know when their inhaler is running low, rather than guessing based on how long they’ve had it.
What Happens When You Press It
Inside the canister, the propellant exists in two phases: liquid and vapor. When you press down on the canister, the metering valve opens and releases a fixed volume of the liquid mixture. The propellant rapidly expands into gas as it hits the lower pressure outside the canister, breaking the medication into a fine mist of tiny droplets. This spray shoots out through the mouthpiece and into your airway. The propellant provides the same vapor pressure whether the canister is full or nearly empty, so each puff delivers a consistent dose.
Modern MDIs use hydrofluoroalkane (HFA) propellants, which replaced older chlorofluorocarbon (CFC) propellants that damaged the ozone layer. The propellant makes up the bulk of the formulation, with the actual drug comprising a small fraction of what’s inside the canister.
Medications Delivered by MDI
MDIs deliver several classes of inhaled medications. The most familiar is albuterol, a fast-acting bronchodilator that opens narrowed airways during an asthma attack or flare-up. MDIs also deliver inhaled corticosteroids that reduce airway inflammation over time, long-acting bronchodilators for daily maintenance therapy, and combination products that pair two drug types in a single device. These medications treat conditions like asthma, COPD, and bronchiectasis.
How to Use an MDI Correctly
Proper technique matters more than most people realize. Studies show that roughly 45% of MDI users make errors with coordination (pressing the canister and breathing in at the same time), 44% inhale at the wrong speed or depth, and 46% skip the breath-hold afterward. Any of these mistakes means less medication reaches your lungs.
The National Heart, Lung, and Blood Institute outlines these steps:
- Prepare: Remove the cap, shake the inhaler, and prime it if needed (usually by spraying a few puffs into the air when new or unused for several days).
- Position: Sit or stand up straight. Breathe out completely to empty your lungs.
- Inhale: Place the mouthpiece in your mouth with a tight seal around it. Press down on the canister as you start to breathe in slowly, continuing that slow, deep breath for 3 to 5 seconds.
- Hold: Hold your breath and count to 10.
- Repeat if needed: Wait about 1 minute between puffs if your prescription calls for more than one.
- Rinse: If your medication is an inhaled corticosteroid, rinse your mouth with water and spit it out to prevent oral infections like thrush.
The key challenge is coordination: you need to press the canister and begin inhaling at the same moment, then continue breathing in slowly and deeply. Breathing in too fast sends the medication crashing into the back of your throat instead of reaching the smaller airways in your lungs.
How Spacers Help
A spacer (sometimes called a valved holding chamber) is a tube that attaches between the MDI and your mouth. When you press the canister, the medication sprays into the spacer first, where it hangs suspended for a moment. This gives you more time to inhale, which largely eliminates the coordination problem. The spacer also slows down the spray so fewer large droplets hit the back of your throat, meaning more medication gets to your lungs. Spacers are especially useful for children, older adults, and anyone who struggles with the press-and-breathe timing.
MDIs Compared to Other Inhalers
Dry powder inhalers (DPIs) contain medication in powdered form with no propellant. Instead of pressing a canister, you load a dose and then breathe in with one quick, forceful inhalation. The force of your breath pulls the powder into your lungs. This eliminates the coordination issue, but DPIs depend heavily on how hard you can inhale. People with severe breathing difficulty or young children may not generate enough airflow to get a full dose.
Soft mist inhalers (SMIs) use a mechanical spring to push liquid medication through a tiny nozzle, creating a slow-moving cloud without any propellant. The slower spray makes coordination easier than with an MDI, and the device doesn’t require as much inspiratory effort as a DPI.
One advantage MDIs hold over both alternatives: the delivered dose is consistent and doesn’t depend on how strongly you breathe in. That makes MDIs particularly useful during acute breathing episodes when inhaling forcefully is difficult.
Cleaning and Maintenance
Medication residue builds up inside the plastic actuator over time and can partially or completely block the spray. Wash the actuator at least once a week by removing the metal canister, running warm water through the top and through the mouthpiece for about 30 seconds, then letting it air-dry completely, ideally overnight. Never wash the metal canister or get it wet.
If you notice weak or no spray coming out, blockage is the likely culprit. This happens more often when the actuator isn’t fully dried between washes. After cleaning and drying, put the canister back in, shake the inhaler, and spray two test puffs into the air before using it again.
Environmental Considerations
The HFA propellants in MDIs are potent greenhouse gases. A regional study in South Tyrol, Italy found that MDIs were responsible for over 1,000 tonnes of CO₂-equivalent emissions annually, compared to roughly 50 tonnes from dry powder inhalers. This difference has led some healthcare systems, particularly in Europe, to encourage switching to DPIs when clinically appropriate. That said, the right inhaler is the one that gets medication into your lungs effectively. For many patients, especially those who can’t generate a strong enough breath for a DPI, an MDI remains the best option.