Add-on devices for pressurised metered dose inhalers (MDIs) improve "targeting" of drug to the lungs and can correct for hand-breath dyscoordination. Measurements of drug delivery from add-on devices by gamma scintigraphy have shown that compared to an MDI, oropharyngeal deposition is always reduced, and that lung deposition is generally either increased or unchanged. The total body dose may be reduced by over 80%. Increases in lung deposition may not result in improved bronchodilator response if the top of the dose-response curve has been reached. Add-on devices with one-way valves and mouthpiece or mask may enable asthma to be controlled with a smaller delivered dose of drug than from an MDI, and have proved to be viable lower cost alternatives to the use of nebulizers for delivering high dose bronchodilators to patients with severe acute asthma, and steroids to chronic asthmatics.