The most effective way of taking most asthma treatments is to inhale the medicine directly into the lungs. Inhalation allows a lower dose to be administered and therefore reduces the incidence of potential side effects. A standard dose of salbutamol for inhalation is 100 micrograms, which compares to an oral dose of 2 or 4mg. Hence if drugs can be delivered directly to the lung, they will act faster and at a lower dose (20 to 40 fold in the case of salbutamol).

Most preventer treatments contain steroids and taking them by inhaler means that a much lower dose of the steroid can be used, and because the inhaled medicine goes straight down to the airways where it is needed, very little is absorbed into the rest of the body. When steroids are taken in tablet form the dose is much higher (one steroid tablet gives 50 times the dosage of a puff from a standard steroid inhaler) and most of it will be absorbed into the rest of the body, not just the lungs.

Using steroid tablets regularly for long periods of time (months or years) can have serious side effects such as osteoporosis, bruising easily, diabetes, cataracts, increased hunger, heartburn and indigestion. They may bring on a depressed feeling, cause mood swings or a fattened face (moon face). Patients using high dose steroid inhalers (more than 800mcg beclometasone dipropionate or equivalent daily) should be supplied with a steroid warning card.