Diphenhydramine

What is this medication for?

• Pruritus (adjunct), motion sickness, mild allergic reaction/anaphylaxis adjunct, pre-transfusion/pre-chemo prophylaxis

How is it given?

Give exactly as your veterinarian prescribed — never change the dose or stop on your own unless your veterinarian has told you to.

What if I miss a dose?

• Give it as soon as you remember — unless it is almost time for the next dose (then skip the missed one; never double up).
• Call your clinic if you miss more than one dose in a row.

Side effects — what should I watch for?

Effects listed as common in this handout

• Sedation, dry mouth (anticholinergic) (Most common)
• Paradoxical excitement (Documented)
• stomach/intestinal effects (vomiting, diarrhea, loss of appetite) (Less common)

Worth a phone call to your veterinarian

• Any new symptom that worries you.

⚠️ Contact your veterinarian or an emergency clinic IMMEDIATELY if you notice

• Trouble breathing, collapse, seizures, severe vomiting or diarrhea, facial swelling, or pale gums

Before starting (or refilling), tell your veterinarian if your pet:

• has or has had: Hypersensitivity to antihistamines in this class
• has or has had: Angle-closure glaucoma, prostatic hypertrophy, bladder-neck/pyloroduodenal obstruction, COPD (if mucus is a problem), hyperthyroidism, cardiovascular disease/high blood pressure

• is pregnant, nursing, or used for breeding
• is taking any other medications, supplements, or flea/tick products, including:
  • Other CNS depressants
  • Anticholinergic drugs (incl. TCAs)

Storage and handling

Room temperature, protect from light/freezing.

Good to know

• Not the best antiemetic choice for cats — histamine is not a major mediator of feline emesis; prefer maropitant or a phenothiazine for that indication in cats.
• Efficacy for itching control alone (without other therapy) is modest at best — set expectations as adjunctive, not primary, therapy.
• Distasteful liquid form — use tablets/capsules where possible for compliance.