Current guidelines emphasize the efficacy of inhaled corticosteroids for anti-inflammatory activity in asthma, and recommend higher doses and earlier initiation of therapy than previous guidelines. Concern over possible side effects with long term use has prompted an evaluation of the available literature to determine the optimal dose that may be administered without fear that significant side effects might occur (e.g., growth retardation in children, adrenal suppression, reduction in bone mineral density, cataract formation). Regular treatment with the following drugs in adults and children, respectively, is unlikely to result in any clinically significant effects on the above parameters: beclomethasone dipropionate less than 1500 micrograms and 400 micrograms, budesonide less than 1600 micrograms and 400 micrograms, flunisolide less than 2000 micrograms and 1000 micrograms, fluticasone propionate approximately 500 micrograms and 200 micrograms, and triamcinolone acetonide less than 1600 micrograms and 1200 micrograms. Systemic effects are influenced by potency and bioavailability. Inhaled corticosteroids owe their favourable safety profile to a high topical to systemic potency ratio compared with that of oral corticosteroids. In terms of relative topical potency, fluticasone propionate is more potent than budesonide, which is more potent than beclomethasone dipropionate, which is more potent than flunisolide and triamcinolone acetonide. The delivery device has an important influence on the amount of drug reaching the patient. A spacer device attached to a metered dose inhaler or a Turbuhaler reduces oropharyngeal deposition and increases lung deposition. As a result, a dosage reduction may be possible, and local side effects of dysphonia and oral candidiasis may be reduced. Patients requiring continued high doses by the inhaled route should be monitored for systemic effects and be considered for osteoporosis prevention therapy if appropriate.