Cholinergic adverse effects of acetylcholinesterase inhibitors (AChEIs) are caused by their central and peripheral pharmacological actions on a variety of organ tissues. Gastrointestinal adverse effects predominate and these were relatively common in the phase II and III randomised clinical trials of AChEIs for the treatment of probable Alzheimer's disease. However, in these studies forced and rapid titration of drugs was used, which is not the case in clinical practice. Although there is a risk of pharmacodynamic interactions with other drugs leading to enhanced cholinergic adverse effects, very few of these interactions have proven to be clinically significant. Unresolved issues include the mechanism of syncope and neuromuscular weakness, which should be resolved through structured pharmacovigilance programmes and clinical studies. Loss of bodyweight may prove to be a long term significant complication. As a class, the AChEIs have proven to be well tolerated in the symptomatic treatment of Alzheimer's disease in its mild-to-moderately severe stages. The incidence and clinical significance of cholinergic adverse events will need to be carefully studied if the drugs are used for indications other than Alzheimer's disease.