Can antihistamines cause erectile disfunction? and is the effect temporary? like while the drug is in your body, or does it linger for days and weeks?

Possible: If the antihistamine makes you drowsy or sedated, then it may affect your sexual performance. However the newer generation antihistamines do not usually cause drowsiness . I would expect the effect to last no longer than 24 hrs (although it may affect allergy skin testing for up to 72 hrs). I have a feeling that your ed is not from the antihistamine however. See an urologist.