A metered dose inhaler (MDI) is a handheld device that delivers a precise, pre-measured spray of medication directly into your lungs. It’s the most common type of inhaler used for asthma and chronic obstructive pulmonary disease (COPD), and it works by using a pressurized propellant to turn liquid medication into a fine mist you breathe in. If you’ve ever seen someone press down on a small canister and inhale from a plastic mouthpiece, that’s an MDI.
How the Device Is Built
An MDI has four main parts that work together. The canister is the small metal cylinder that holds the medication mixed with a pressurized propellant. It sits upside down inside a plastic actuator (sometimes called the boot or housing), which is the L-shaped piece you hold in your hand. The actuator has a mouthpiece at one end and a small nozzle inside that shapes the spray into a fine mist as it leaves the canister.
Inside the canister, a metering valve measures out exactly the same amount of medication every time you press down. This is the “metered dose” part of the name. Each press releases one consistent puff regardless of how full or nearly empty the canister is. Most modern MDIs also have a dose counter built into the device, typically counting down from the total number of doses to zero so you know when it’s time for a replacement.
What Happens When You Press It
The bulk of what’s inside the canister is actually propellant, not medication. Modern MDIs use hydrofluoroalkane (HFA) propellants, which are gases compressed into liquid form. When you press the canister down, the metering valve opens and releases a tiny, pre-measured volume of the liquid formulation through the nozzle. As it hits the lower pressure of the outside air, the propellant rapidly expands and evaporates, breaking the medication into a cloud of very fine droplets.
The nozzle’s size and shape control how fast the spray travels and how small the droplets are. Both matter because particles need to be small enough to travel past your throat and into your lower airways, where they do their work. With perfect technique, anywhere from 8% to 53% of the medication actually reaches your lungs. The rest lands in your mouth and throat or stays in the device.
Types of Medication Delivered
MDIs deliver several classes of inhaled medication. Short-acting bronchodilators (often called rescue inhalers) relax the muscles around your airways within minutes and are used for quick relief during an asthma attack or sudden shortness of breath. Long-acting bronchodilators work on the same principle but are designed to keep airways open for 12 hours or more as a daily maintenance treatment.
Inhaled corticosteroids reduce inflammation in the airways over time and are the cornerstone of daily asthma control. These don’t provide instant relief but prevent flare-ups when used consistently. Combination inhalers pack both a long-acting bronchodilator and a corticosteroid into a single MDI so you get both effects from one device.
How to Use an MDI Correctly
Technique matters more than most people realize. Poor coordination between pressing the canister and breathing in is the single most common mistake, and it can dramatically reduce how much medication reaches your lungs. The National Heart, Lung, and Blood Institute recommends this sequence:
- Prepare: Remove the cap and shake the inhaler well. If it’s new or hasn’t been used in a while, prime it by spraying a few test puffs into the air (check your specific inhaler’s instructions for how many).
- Exhale: Breathe out completely to empty your lungs before bringing the inhaler to your mouth.
- Inhale and activate together: Place the mouthpiece in your mouth with a tight seal around it. As you start to breathe in, press down firmly on the canister to release one puff. Continue breathing in slowly for 3 to 5 seconds, taking as deep a breath as possible.
- Hold: Remove the inhaler from your mouth and hold your breath for a count of 10. This gives the medication time to settle into your airways.
- Wait between puffs: If you need a second puff, wait about 1 minute before repeating the process.
The slow, steady inhalation is key. Breathing in too fast sends the medication crashing into the back of your throat instead of carrying it deep into your lungs.
Why a Spacer Makes a Difference
A spacer (also called a valved holding chamber) is a tube or chamber that attaches between the MDI and your mouth. It contains a one-way valve that holds the medication cloud inside the chamber until you breathe in, then prevents your exhaled breath from pushing back into it.
Spacers solve two problems at once. First, they eliminate the need to perfectly time your breath with the canister press. The medication simply waits in the chamber for you to inhale, which is why they’re especially useful for young children, older adults, and anyone who struggles with the coordination required for bare MDI use. Children under 3 typically use a spacer with a face mask attached.
Second, spacers slow down the spray and filter out larger particles before they reach your mouth. This reduces the amount of medication that deposits in your throat (which can cause side effects like oral thrush with steroid inhalers) and increases the amount that reaches your lungs. Studies show lung deposition rates of 11% to 68% with a spacer, compared to 8% to 53% without one. That upper range nearly doubles. For this reason, many clinicians recommend spacers for all MDI users, not just those with coordination difficulties.
Priming and Dose Tracking
Priming means spraying a few puffs into the air before using the inhaler on yourself. You need to prime when the inhaler is brand new, when it hasn’t been used for several days, or if it’s been dropped. This clears the valve and ensures the first dose you inhale contains the correct amount of medication. The number of priming sprays varies by product, so check the instructions that come with yours.
Most current MDIs have built-in dose counters that count down from the total number of available doses toward zero. The FDA recommends these counters provide a clear warning when the inhaler is getting low, giving you enough time to refill your prescription before it runs out completely. Some older or generic inhalers use color-coded windows instead of numbers: green means plenty of doses remain, yellow means it’s getting low, and red means it’s time to replace it. If your inhaler lacks any counter, you can track doses by recording each use, but floating the canister in water to check fullness is unreliable and no longer recommended.
Cleaning and Maintenance
The plastic actuator should be cleaned at least once a week to prevent medication buildup from clogging the nozzle. A partially blocked nozzle changes the spray pattern and can reduce how much medication you actually inhale. For most rescue inhalers, the process is straightforward: remove the metal canister (never submerge it in water), then run warm water through the top of the actuator and out through the mouthpiece for about 30 seconds each way. Let all the pieces air dry completely, usually overnight, before reassembling.
Some steroid inhalers have different instructions. Rather than rinsing the whole actuator, you may only need to clean the small opening where the spray exits using a damp cotton swab, then wipe the inside of the mouthpiece with a damp tissue. Always check the cleaning method for your specific inhaler, because getting the wrong component wet can damage it or affect the next dose.