First-generation antihistamines

What is this medication for?

• Pruritus/allergy adjunct (atopy trial, flea-allergy dermatitis); hydroxyzine = anxiolytic-antihistamine hybrid; chlorpheniramine = mast-cell-tumor mediator control adjunct (4–8 mg/dog)

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/low energy/tiredness (Main effect (sometimes desired))
• Anticholinergic: dry mouth, urine retention, stomach/intestinal (Class)
• Paradoxical excitement (Cats)

Worth a phone call to your veterinarian

• Any new symptom that worries you.

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

• Overdose: wobbliness/unsteadiness→seizures→coma (Narrow-ish margin in small dogs)

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

• has or has had: Glaucoma, prostatic hypertrophy, urinary retention, cardiac disease (anticholinergic); MAOI co-use

• is pregnant, nursing, or used for breeding
• is taking any other medications, supplements, or flea/tick products, including:
  • Other anticholinergics/CNS depressants
  • Apomorphine note
  • Ketoconazole (human, terfenadine-era)

Storage and handling

OTC; chlorpheniramine 4 mg tablets — the compounding-friendly size; toxicity note: 25 mg/kg ingestion killed a dachshund (11 h — wobbliness/unsteadiness→coma→arrest) — keep counted in multi-pet homes.

Good to know

• Hydroxyzine is cetirizine's parent with a drowsiness engine — when the sedative antihistamine effect is wanted (anxious itchy dog at night), hydroxyzine; when not, use the metabolite (cetirizine — see cetirizine.md).
• Chlorpheniramine's second job: MCT mediator-masking adjunct (H1 + partial mast-cell stabilization) — the historical protocol partner of the H2 blocker.
• Cat + antihistamine = excitement lottery: warn the owner before the first dose.