Taking too many antihistamines than needed can cause side effects such as trouble urinating, Vomiting, Blurred Vision, poor coordination, dry mouth and dizziness.
A few other symptoms that you might experience when taking too many antihistamines are lightheadedness, dry nose, loss of appetite and Gastrointestinal upset which includes urinary constipation and vomiting.
Taking too many antihistamines and other medicine could also cause liver damage or kidney problems as well.