ASK DIS: Interaction: Benadryl and Antihistamine

Mechanism of action

Diphenhydramine provide symptomatic relief in cases of allergic cough by reducing the body’s production of histamine
Ammonium chloride and ammonium carbonate have an irritant effect on mucous membranes and are considered to have expectorant properties

Interactions

Level of interaction: Moderate
Usually avoid combinations; use it only under special circumstances
Excessive parasympatholytic effects may result in paralytic ileus, hyperthermia, heat stroke, and the anticholinergic intoxication syndrome.
Peripheral symptoms of intoxication commonly include mydriasis, blurred vision, flushed face, fever, dry skin and mucous membranes, tachycardia, urinary retention, and constipation.
Central symptoms may include memory loss, disorientation, incoherence, hallucinations, psychosis, delirium, hyperactivity, twitching or jerking movements, stereotypy, and seizures.
Central nervous system-depressant effects may also be additively or synergistically increased when these agents are combined, especially in elderly or debilitated patients

Second Generation Antihistamine

example: Loratidine, Cetirizine
Benadryl and loratidine are both antihistamines and work similarly in the body
loratidine is long-acting drug, so if they are taken together their effects will overlap and patients may duplicate the active drug of each medication
If patients have uncontrolled allergies, combination of Benadryl and loratidine may be a good solution, but only if they are taken in different times of a day.
loratidine should be taken in the morning due to its long-acting and no-sedative properties while Benadryl should be taken at night since it has sedative properties, so it can help patients with uncontrolled allergy symptoms (such as unbearable itches) to fall asleep easier.
The table below shows the incidence of side effects after Benadryl and loratidine administration in recommendable doses. The incidence can be increased if these drugs are overdosed or taken together.

Incidence	Side effects
10-15% (very common)	Headache
1-10% (common)	Nervousness, Malaise, Somnolence, Drowsiness, Fatigue, Dry mouth, Hyperkinesia, Diarrhea, Dysphonia, Abdominal pain, Conjunctivitis, Dysmenorrhea, Dizziness, Pain in extremity, Somnolence, Rhinorrhea, Pyrexia, Stomach discomfort
Frequency Not Defined	Xerostomia, Confusion, Dry nasal mucosa, Pharyngeal dryness, Agranulocytosis, Thrombocytopenia, Convulsions, Hypotension, Palpitations, Urinary retention, Neuritis, Diplopia, Tinnitus, Arthralgia, Myalgia, Hemolytic anemia,

Recommendations:

Usually avoid combinations; use it only under special circumstances
If there is no heart, blood pressure, urinary or kidney contraindications and the use is indicated (severe allergic reactions) this combination will not likely be a problem.
Monitor for signs of overdose / side effects

References:

ASK DIS