Phenobarbital (phenobarbitone) and phenytoin are the most useful anticonvulsants in neonates because adverse effects are most readily reversed when these drugs are used. Most anticonvulsants are very rarely associated with haematological adverse effects. Platelet function is particularly vulnerable to valproic acid (sodium valproate) therapy. Barbiturates and phenytoin can precipitate metabolic bone disease. Although very infrequent, lymphadenopathy is most common with phenytoin, and lupus-like illnesses with ethosuximide. Valproic acid may precipitate underlying metabolic disorders. Nephrolithiasis can occur with topiramate. Liver disease is most likely with felbamate or valproic acid, but can occur with other anticonvulsants. Valproic acid and ethosuximide are the main precipitants of gastrointestinal symptomatology; while valproic acid and vigabatrin are frequently associated with excessive bodyweight gain. Rash is most likely to occur with barbiturates, but there is a high risk of this adverse effect if large doses of lamotrigine are given with valproic acid. Adverse cosmetic effects are most likely with phenytoin, but valproic acid may cause alopecia. All anticonvulsants may cause unwanted neurological effects: when they occur, diplopia is usually precipitated by carbamazepine; tremor by valproic acid; and other motor disturbances are probably most common with phenytoin. Most anticonvulsants can cause drowsiness. Phenobarbital leads anticonvulsants as a cause of behavioural difficulties. Effects of anticonvulsants on cognitive function are difficult to assess, but subtle changes have been reported for all anticonvulsants in use up to the 1980s. Compared with other anticonvulsant drugs, phenytoin and felbamate are more often discontinued as a result of unwanted effects.