Preclinical and clinical studies have clearly demonstrated that zanamivir, a potent and highly selective inhibitor of the influenza A and B virus neuraminidase, has an impressive safety profile. This report describes the safety and tolerability findings from the clinical studies completed up to the 17 July 1998 involving over 6000 adult and adolescent patients from North America, Europe and the Southern Hemisphere. Serious adverse events from an ongoing Japanese clinical programme are also reported. Zanamivir was administered in various dose forms and frequencies and was found to have a comparable safety profile with placebo when given for both the treatment and prophylaxis of influenza-like illness. These findings were independent of age and underlying medical condition. 4152 patients received zanamivir and the most commonly reported adverse events were consistent with the signs and symptoms of influenza-like illness. Most of the adverse events were mild and did not result in patient withdrawal from the studies. Less than 1% of zanamivir and placebo recipients reported a serious adverse event. In addition, 490 healthy volunteers received zanamivir in clinical pharmacology studies. It was well tolerated and the incidence of adverse events was similar in zanamivir and placebo recipients. In addition, no clinically significant laboratory abnormalities were detected. Results from in vitro and in vivo animal studies suggest that zanamivir has low acute toxicity and no significant systemic toxicity or respiratory tract irritancy at plasma exposures more than 100-fold higher than those anticipated following clinical use. Neither genotoxic nor reproductive types of toxicity have been observed in toxicology studies at doses equal to 17 to 197 times the current therapeutic dose (20 mg/day). The characteristics of the molecule and the low systemic exposure indicate a very low potential for drug interactions with the inhaled route. Furthermore, repeated 600mg intravenous doses were well tolerated in healthy volunteers. The observed safety profile of zanamivir compares favourably with currently available agents with anti-influenza virus activity, such as rimantadine and amantadine, as well as GS4104, a neuraminidase inhibitor currently in phase III development. This may be attributed to the low systemic bioavailability of zanamivir, which is given by oral inhalation, direct to the primary site of viral replication. The potential advantages of this include a reduced risk of drug-drug interactions, other nontarget organ toxicities (e.g. brain) and drug clearance issues from both kidney and liver. Therefore, the safety profile of zanamivir supports its use in the management of influenza.