# Diphenhydramine: veterinary dose and monograph

> Diphenhydramine is a veterinary drug. First-generation H1 antihistamine with marked sedative and antiemetic / anti-motion-sickness action. Indications include allergic pruritus / urticaria, atopy (adjunct); pre-treatment for mast-cell-tumour handling / transfusion / vaccine reactions; motion sickness / mild travel sedation. This monograph lists 8 reference doses for dogs, cats, cattle, horses, rabbits and ferrets and 1 more species, each with route, frequency and source, plus meat and milk withdrawal periods for food animals.

Source: https://vetmasterji.com/drugs/diphenhydramine · Last reviewed: 2026-10-01

## At a glance

- **Drug class:** Veterinary drug
- **Also known as:** Benadryl, Dimedrol
- **Species with doses:** Dogs, Cats, Cattle, Horses, Rabbits, Ferrets and Primates
- **General dose range:** 1–4 mg/kg PO q8–12h — check the species tables for the indication
- **Routes:** PO, IM
- **Last reviewed:** 1 October 2026

## Indications

- Allergic pruritus / urticaria, atopy (adjunct)
- Pre-treatment for mast-cell-tumour handling / transfusion / vaccine reactions
- Motion sickness / mild travel sedation

## Diphenhydramine dose by species

Doses are per administration unless stated. mg/kg doses scale with body weight; per-animal, per-quarter and infusion-rate doses are shown as the reference writes them.

### Dogs

*Diphenhydramine doses for dogs*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Antihistamine / mild sedation | 2–4 mg/kg | PO | q8–12h | — |
| Pre-medication for MCT handling / reaction (parenteral) | 1–2 mg/kg | IM | once before procedure | — |

- Antihistamine / mild sedation: Plumb’s p.420: 2–4 mg/kg PO q8–12h (Papich 2000); 2.2 mg/kg PO two to three times daily; atopic dermatitis 25–50 mg per dog PO three times daily; motion sickness 2–4 mg/kg PO or IM q8h. Papich 5e: 2.2 mg/kg q8–12h PO, IM or SC (25–50 mg per large dog).
- Pre-medication for MCT handling / reaction (parenteral): Plumb’s p.420: 1 mg/kg IM, SC or IV q8–12h, not exceeding 40 mg total (Papich 2000); urticaria/angioedema 2 mg/kg IM twice daily; doxorubicin anaphylaxis 3–4 mg/kg IM with dexamethasone. Papich 5e: 2.2 mg/kg IM or SC. Give IV slowly — rapid IV can cause hypotension.

### Cats

*Diphenhydramine doses for cats*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Antihistamine | 2–4 mg/kg | PO | q8h | — |

- Antihistamine: Papich 5e: 2–4 mg/kg q6–8h PO, or 1 mg/kg q8h IV or IM. Plumb’s p.420: 2–4 mg/kg PO q8h (Scherk 2006); lower regimens 0.5 mg/kg PO q12h (Messinger) or 2–4 mg per cat q12–24h (Hnilica). The liquid is distasteful to cats.

### Cattle

*Diphenhydramine doses for cattle*

| Indication | Dose | Route | Frequency | Duration | Withdrawal |
| --- | --- | --- | --- | --- | --- |
| Anaphylaxis adjunct | 0.5–1 mg/kg | IM | as needed | — | Withdrawal NOT established — treat as extra-label |

- Anaphylaxis adjunct: Plumb’s p.420 (Clark 1986): 0.5–1 mg/kg IM or IV with epinephrine and corticosteroids. Papich 5e: 0.5–1 mg/kg IM. Extra-label — obtain a FARAD withdrawal.

### Horses

*Diphenhydramine doses for horses*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Anaphylaxis adjunct | 0.25–1 mg/kg | IM | as needed | — |

- Anaphylaxis adjunct: Plumb’s p.420 (Evans 1996): 0.25–1 mg/kg IV or IM. Papich 5e (large animals): 0.5–1 mg/kg IM as a single dose as needed. Allergic skin disease: 0.75–2 mg/kg PO q12h (Rees; Miller). ARCI Class 3.

### Buffalo

No buffalo-specific doses are on file, so buffalo are dosed as cattle.

*Diphenhydramine doses for buffalo*

| Indication | Dose | Route | Frequency | Duration | Withdrawal |
| --- | --- | --- | --- | --- | --- |
| Anaphylaxis adjunct | 0.5–1 mg/kg | IM | as needed | — | Withdrawal NOT established — treat as extra-label |

- Anaphylaxis adjunct: Plumb’s p.420 (Clark 1986): 0.5–1 mg/kg IM or IV with epinephrine and corticosteroids. Papich 5e: 0.5–1 mg/kg IM. Extra-label — obtain a FARAD withdrawal.

### Rabbits

*Diphenhydramine doses for rabbits*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Antihistamine | 1–2 mg/kg | PO | q12h | — |

- Antihistamine: Plumb’s p.420 (Morrisey & Antinoff 2003): rabbits 1–2 mg/kg PO twice daily.

### Ferrets

*Diphenhydramine doses for ferrets*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Pre-vaccination (prevent anaphylaxis) | 2 mg/kg | IM | once | — |

- Pre-vaccination (prevent anaphylaxis): Plumb's p.421 (Williams 2000): 2 mg/kg PO, IM or IV given ~10 minutes before vaccination. Ferrets are prone to vaccine anaphylaxis — pre-treatment is routine.

### Primates

*Diphenhydramine doses for primates*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Antihistamine (monkeys, lemurs) | 5 mg/kg | PO | q6–8h | — |

- Antihistamine (monkeys, lemurs): Carpenter Exotic Animal Formulary 6th ed, Table 13-7 (p.705): monkeys/lemurs 5 mg/kg/day PO/IM/IV divided q6–8h; chimpanzees 25–50 mg/animal PO/IM/IV q6–8h.

## Contraindications

Caution in glaucoma, urinary retention, prostatic disease and significant cardiac disease. Additive CNS depression with other sedatives. Avoid the rapid IV route (hypotension); IM is preferred for parenteral use.

## Adverse effects

- Sedation, lethargy
- Anticholinergic: dry mouth, urinary retention, GI slowing
- Paradoxical excitement (occasional, especially cats)

## Drug interactions

- Additive CNS depression with other sedatives / opioids / anaesthetics
- Additive anticholinergic effects with atropine-like drugs
- Blunts the wheal of intradermal allergy testing — withdraw before testing
- Monoamine-oxidase inhibitors: intensified anticholinergic effects

## Pregnancy and lactation

Regarded as relatively safe and used where antihistamine therapy is needed, but use only if required.

## Monitoring

- Sedation level and demeanour
- Heart rate; urination (anticholinergic retention)

## Toxicity and overdose

Overdose: initial CNS depression or paradoxical excitation, anticholinergic signs (mydriasis, dry mucous membranes, urinary retention, ileus), tachycardia and, in large ingestions, seizures.

Management: Supportive — control seizures (diazepam), manage tachyarrhythmia, IV fluids and cooling; physostigmine may reverse severe central anticholinergic toxicity under monitoring. Decontaminate recent large ingestions.

## Clinical pearls

- Antihistamines work best as prevention — give before the allergen / mast-cell handling, not after histamine is already released
- Give IM rather than fast IV; rapid intravenous injection can drop blood pressure
- Use a plain diphenhydramine product — combination cold remedies often contain toxic decongestants or paracetamol

## Mechanism of action

First-generation H1-antihistamine. Competitively blocks histamine H1 receptors to reduce the vasodilation, oedema and pruritus of allergic responses; its lipophilicity allows CNS penetration (sedation, antiemetic / anti-motion-sickness effect) and it has anticholinergic activity.

## Formulations and products

- Diphenhydramine 25 mg tablet / capsule
- Diphenhydramine 50 mg/mL injection
- Diphenhydramine 2.5 mg/mL elixir
- Benadryl — 25 / 50 mg capsule; syrup, Capsule / syrup (human-label)

## Storage

Store at 15–30 °C; protect elixir / injection from light.

## Before you use this dose

> **Caution:** Reference doses for veterinary professionals. Confirm the dose against the product label, the patient’s condition, current guidance and local regulations before use; the attending veterinarian remains responsible for every clinical decision.

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## Frequently asked questions

### What is the dose of Diphenhydramine for dogs?

Reference doses for Diphenhydramine in dogs: 2–4 mg/kg PO q8–12h (Antihistamine / mild sedation); 1–2 mg/kg IM once before procedure (Pre-medication for MCT handling / reaction (parenteral)). Confirm against the label and the patient before use.

### What is the dose of Diphenhydramine for cats?

Reference doses for Diphenhydramine in cats: 2–4 mg/kg PO q8h (Antihistamine). Confirm against the label and the patient before use.

### What is the dose of Diphenhydramine for cattle?

Reference doses for Diphenhydramine in cattle: 0.5–1 mg/kg IM as needed (Anaphylaxis adjunct). Confirm against the label and the patient before use.

### What is the withdrawal period for Diphenhydramine?

Cattle, IM (Anaphylaxis adjunct): Withdrawal NOT established — treat as extra-label. Follow the product label and national regulations where they differ.

### When should Diphenhydramine not be used?

Caution in glaucoma, urinary retention, prostatic disease and significant cardiac disease. Additive CNS depression with other sedatives. Avoid the rapid IV route (hypotension); IM is preferred for parenteral use.

### What are the side effects of Diphenhydramine in animals?

Sedation, lethargy; Anticholinergic: dry mouth, urinary retention, GI slowing; Paradoxical excitement (occasional, especially cats).

## Sources

- Plumb's Veterinary Drug Handbook
- Papich, Papich Handbook of Veterinary Drugs, 5th edition
- Carpenter, Exotic Animal Formulary

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VetMasterJi is a clinical decision-support and education platform for veterinary professionals and students. It does not provide veterinary advice, diagnosis or treatment for specific animals. All drug doses, calculators and differentials are references that must be independently verified before clinical use; the attending registered veterinarian remains solely responsible for every clinical decision.