The standard MDI inhaler (shown on the right)
This has been used for over 40 years. This type of inhaler is used to deliver various types and brands of drugs. The MDI contains a pressurised inactive gas that propels a dose of drug in each 'puff'. Each dose is released by pressing the top of the inhaler. This type of inhaler is quick to use, small, and convenient to carry. It needs good co-ordination to press the canister, and breathe in fully at the same time.
The standard MDI is the most widely used inhaler. However, many people do not use it to its best effect. Common errors include: not shaking the inhaler before using it; inhaling too jerkily or at the wrong time; not holding your breath long enough after breathing in the contents.
Breath-activated MDIs
These are alternatives to the standard MDI. For example, the autohaler shown on the right. You don't have to push the canister to release a dose. Instead, you trigger a dose by breathing in at the mouthpiece. So, these types of MDI inhalers require less co-ordination than the standard MDI. They tend to be slightly bigger than the standard MDI.
Until recently, the propellant gas in MDI inhalers has been a CFC (chlorofluorocarbon). However, CFCs damage the earth's ozone layer, and so are being phased out. The newer CFC-free inhalers work just as well, but they use a different propellant gas that does not damage the ozone layer.