Calculated using published guidelines from AVMA, ASPCA, AAHA, and NRC standards.
This dog OTC medication calculator provides oral diphenhydramine and famotidine reference ranges and converts veterinarian-provided doses into tablet or liquid equivalents. Cetirizine and melatonin require a veterinarian-entered dose. A calculated number cannot determine which medication is suitable for your dog.
Dog OTC dose and product-conversion calculator
Use a veterinarian-provided dose to calculate tablet or oral-liquid equivalents. Diphenhydramine and famotidine also offer published reference ranges. A reference range does not select a treatment or establish a safe dose for your dog.
Suspected overdose, collapse, seizures or trouble breathing: contact an emergency veterinarian immediately. For suspected poisoning in the United States, call ASPCA Poison Control: 888-426-4435 or Pet Poison Helpline: 855-764-7661. Fees may apply.
Dogs only; oral products only. This tool does not check all diseases, interactions or contraindications. Your veterinarian must choose the medication, formulation, dose, interval and treatment duration. Read the calculation methods and sources.
Medication methods and source register
All doses below refer to a single oral dose for a dog, not a daily total. The calculator uses kilograms internally; 1 lb = 0.45359237 kg. Reference ranges describe the cited indications and are not universal safety limits.
| Medication | Method | Source and scope |
|---|---|---|
| Diphenhydramine (Benadryl) | Reference amount = weight in kg × 2–4 mg/kg per dose. Source interval: every 8–12 hours. A veterinarian-entered dose overrides this reference calculation. | Merck antihistamine table for skin disease; the calculator covers the oral route only. |
| Famotidine (Pepcid AC) | Reference amount = weight in kg × 0.5–1 mg/kg per dose. Source interval: every 12–24 hours. A veterinarian-entered dose overrides this reference calculation. | Merck gastrointestinal-ulcer guidance; the calculator covers the oral route only. |
| Cetirizine (Zyrtec) | Enter your veterinarian’s total mg per dose; the calculator converts it to the selected product quantity. No automatic 20 mg cap or schedule is applied. | VCA cetirizine guidance directs owners to follow the veterinarian’s instructions. The entered prescription is the dosing source. |
| Melatonin | Enter your veterinarian’s total mg per dose; the calculator converts it to the selected product quantity. No automatic weight tiers or schedule are applied. | VCA melatonin guidance. The entered prescription is the dosing source. |
These ranges are not interchangeable with every published protocol. For example, AAHA’s allergic-skin-disease table uses a different diphenhydramine range and notes questionable oral efficacy. A veterinarian must choose the indication, dose, frequency and duration; the calculator does not select between clinical protocols.
Tablet and liquid conversion: what the numbers mean
- Tablets per dose = prescribed mg per dose ÷ mg per tablet.
- Liquid mL per dose = prescribed mg per dose ÷ label mg × label mL.
For a liquid labeled 12.5 mg per 5 mL, enter 12.5 as the drug amount and 5 as the label volume. Do not assume another brand or formulation has the same concentration. The calculation uses full precision, with displayed equivalents rounded to at most three decimal places.
Correction to the earlier table: 10 mg is 0.4 of a 25 mg tablet; half of a 25 mg tablet contains 12.5 mg. This is an arithmetic example, not a dosing recommendation. Decimal equivalents do not establish whether a tablet can be split accurately. Ask your veterinarian or pharmacist to confirm the formulation and practical measuring method.
The calculator no longer rounds to quarter-tablets, teaspoons or “a few drops.” For liquids, use the oral measuring device and precision specified by your veterinarian or pharmacist. For tablets, do not turn a decimal equivalent into a cutting instruction yourself.
Product checks and situations requiring veterinary assessment
Check both the active and inactive ingredients. A familiar brand name does not guarantee the same formulation. The calculator does not assess combination products, extended-release products, injections, cats or other species.
- Diphenhydramine: pediatric animals and dogs with glaucoma, urinary or intestinal obstruction, heart problems or certain other conditions need particular attention. Sedating medicines can also matter. See the VCA medication precautions.
- Cetirizine: cetirizine/pseudoephedrine combinations such as Zyrtec-D are not suitable substitutes. Kidney, liver, heart and seizure conditions, pregnancy and other medicines can affect the decision. See VCA’s cetirizine precautions.
- Famotidine: kidney, liver or heart disease, older age, pregnancy and drug interactions require veterinary consideration. Famotidine is not a general solution for every vomiting episode. See VCA’s famotidine guidance.
- Melatonin: discuss young age, breeding, pregnancy, liver disease and other medicines with your veterinarian. Human products can include xylitol or 5-HTP; these introduce risks beyond melatonin itself. See VCA’s supplement guidance and Merck’s sleep-aid toxicology reference.
This is not a complete contraindication or interaction screen. Provide your veterinarian with your dog’s medical history and every medication and supplement before using an OTC product.
When to seek urgent help
Suspected overdose or poisoning requires immediate advice from a veterinarian or poison-control service; do not wait for a calculator result. Collapse, seizures and difficulty breathing need emergency veterinary care. Keep the packaging available and report the product, amount, time of exposure, weight and symptoms.
United States contacts: ASPCA Poison Control, 888-426-4435; Pet Poison Helpline, 855-764-7661. Both provide 24-hour assistance; fees may apply.
Questions about OTC dose calculations
Does a result mean this medicine is safe for my dog?
No. A calculation checks arithmetic under stated assumptions. It cannot establish diagnosis, product suitability, interactions or an appropriate treatment plan.
Why do cetirizine and melatonin need a veterinarian’s dose?
The previous fixed cap and weight tiers were not supported as universal protocols. These options remain available for converting an individualized dose into product quantities.
Can I switch from pounds to kilograms?
Yes. The tool converts the entered weight and clears the old result. Recalculate after changing any entry. Changing medications also clears the previous prescription and product strength.
Why is my calculated tablet amount a decimal?
It is the exact arithmetic relationship, rounded only for display. A pharmacist or veterinarian must decide whether that amount can be measured with the actual product.
📊 Quick Reference: Standard Canine OTC Medication Dosages
Calibrated against veterinary pharmacology references (Plumb's & Merck Veterinary Manual):
| Medication | Standard Dosage | Frequency | Primary Clinical Use |
|---|---|---|---|
| Benadryl (Diphenhydramine) | 1 mg per 1 lb (2.2 mg/kg) | Every 8–12 hrs | Allergies, insect stings, mild sedation |
| Zyrtec (Cetirizine) | 0.5 mg per 1 lb (max 10mg) | Every 24 hrs | Atopic dermatitis, environmental itch |
| Pepcid AC (Famotidine) | 0.25–0.5 mg per 1 lb | Every 12–24 hrs | Acid reflux, gastritis, nausea |
| Melatonin | 1–6 mg total (by size tier) | Every 8–12 hrs | Thunderstorm anxiety, sleep regulation |
