The rationale for specialised oral formulations of drugs include prolongation of effect for increased patient convenience and reduction of adverse effects through lowered peak plasma concentrations. Local and systemic adverse effects due to high concentrations of drug can be minimised by the use of controlled release delivery systems. Local effects in the gastrointestinal (GI) tract from the release of irritant drug molecules can also be reduced, but the gastric damage caused by nonsteroidal anti-inflammatory drugs (NSAIDs) is only partially relieved by formulation approaches because of the involvement of systemic factors in the aetiology of GI adverse events. The advantages for each drug class must be examined. Newer dosage forms include: (i) osmotic pumps and zero order kinetics systems to control the release rate of the drug; (ii) bioadhesive systems and gastric retention devices to control GI transit; (iii) bioerodible hydrogels; (iv) molecular carrier systems (e.g. cyclodextrin-encapsulated drugs) to modulate local toxicity in the GI tract; (v) externally activated systems; and (vi) colloidal systems such as liposomes and microspheres. There is evidence for improved tolerability for a variety of drugs administered in novel delivery systems. However, the evidence for improved tolerability is complicated by the potential bias in adverse reaction reporting systems, and a lack of studies directly comparing conventional and modified release preparations. The technology now available to produce delivery systems which not only release drugs in a controlled and predetermined fashion, but which can also target to regions of the GI tract such as the colon, should allow greater control of therapy and potentially might minimise patient variables. However, the problem of variable GI transit times still eludes solution. Systems which rely on time to release drug might be more vulnerable to patient-to-patient variability than those which respond to local environments. The effect of food intake is more apparent on single-unit, nondisintegrating dosage forms, although of course none so far are immune from influence. The risk of new adverse effects resulting from such positional therapy with novel delivery devices must be considered. Understanding the mechanisms of induction of individual adverse effects can lead to advances in modes of delivery to decrease the potential for adverse reactions and events while maintaining therapeutic efficacy. Increased compliance can led to increased therapeutic control and hence safety. Each system has to be considered on its merits. No generalisations can be made, although invariably the modulation of high peak plasma concentrations diminishes adverse effects due to rapid absorption.