Antihistamines can cause drowsiness, thickening of mucous, blurred vision, confusion, difficult or painful urination, dizziness, dryness of mouth, nose or throat, fast heartbeat, increased sensitivity of skin to sun, increased sweating, loss of appetite, paradoxical reactions (nightmares, unusual excitement, nervousness, restlessness or irritability), ringing or buzzing in ears, skin rash and stomach upset or pain.
Ammonium salts are irritant to the gastric mucosa, and may produce nausea and vomiting particularly in large doses.