How long once you stop taking symmetrel do the side affects stop?

My Dad has Parkinsons and was subscribed Symmetrel he only took it for a few days but started having nightmares, breathless and is quite confused (thinks hes been out of hospital in a van, walking along corridors in the hospital) which clearly untrue as he has problems with even a couple of steps. he has now been taken off the tablets but is still very confused and saying some really odd things! is this because the tablets are still in his system? if so how long will it take to get them out his system and the side effects to disappear?
Kath Kitson
Very concerned daughter!