Yes you can OD or overdose on antihistamines so you should not take more than your doctor recommends.
It's generally safe to take up to 2 doses of antihistamines per day and anymore than 2 doses or more than 5 or 6 milligrams can mean an overdose in some people.
Taking more antihistamines than you can safely take can lead to health issues and possible death.
It's generally safe to take up to 2 doses of antihistamines per day and anymore than 2 antihistamines per day could cause an overdose.
As for the milligrams for dosage of antihistamines you should take no more than 5 or 6 milligrams of antihistamines or it could lead to an overdose of the antihistamines.
For me I take usually 1 antihistamine per day unless I have more severe allergies and the one antihistamine dosage is not working.
Then in that case I sometimes need to take 2 antihistamines to help with the allergies but my doctor told me I should not take more than 2 antihistamines or it could lead to an overdose.
As long as you don't take more than 2 antihistamines per day you'll be safe and if you do take more than 2 antihistamines you should see a doctor or go to urgent care if you notice any complications.