Antimuscarinics may modestly reduce the rate and possibly the extent of
levodopa absorption. One case describes
levodopa toxicity, which occurred after the withdrawal of an antimuscarinic.
Concurrent use is of established benefit. The presence of a dopa-decarboxylase inhibitor would be expected to minimise the effects of any interaction. There is certainly no need to avoid concurrent use, but it would be prudent to be alert for any evidence of a reduced
levodopa response if
antimuscarinics are added, or for
levodopa toxicity if they are withdrawn.