Zoletil 100 vs Zoletil 50: Dosage Guide for Dogs & Cats, How It Works, Contraindications & What Every Vet Needs to Know

Zoletil is one of the most versatile injectable anaesthetics available in veterinary practice — a Virbac product combining tiletamine (a dissociative NMDA antagonist) and zolazepam (a benzodiazepine) in a fixed 1:1 ratio. It is used as a sole anaesthetic for short procedures, as an induction agent prior to inhalant maintenance, and as a chemical restraint tool for fractious animals and wildlife.

But Zoletil comes with a set of critical safety considerations that are frequently under-appreciated, particularly around drug interactions (the acepromazine combination is genuinely dangerous), contraindications in specific species and conditions, and the important differences between the 100 and 50 formulations. This guide covers all of it.

Zoletil 100 vs Zoletil 50: What Is the Actual Difference?

This is the question most commonly asked about Zoletil, and the answer is simple: the active ingredients are identical in both (tiletamine and zolazepam in a 1:1 ratio), but the concentration after reconstitution with the included 5ml sterile water diluent is different:

When to choose Zoletil 100: Larger dogs, high-dose procedures, field/wildlife use where injecting small volumes is practically important. At 10 mg/kg in a 30kg dog using Zoletil 100, you inject 3ml. With Zoletil 50, you’d need 6ml — twice the volume, which is less practical for IM injection.

When to choose Zoletil 50: Cats and small dogs, where the lower concentration allows more precise measurement at small volumes. At 10mg/kg in a 4kg cat using Zoletil 50, you inject 0.8ml — a manageable volume. Using Zoletil 100, you’d only need 0.4ml, which is difficult to measure accurately with standard syringes.

How Zoletil Works: Tiletamine + Zolazepam Synergy

Understanding why the combination works better than either drug alone helps explain Zoletil’s clinical profile.

Tiletamine is an NMDA (N-methyl-D-aspartate) receptor antagonist — the same receptor class targeted by ketamine, but with a longer duration of action. It causes dissociative anaesthesia: sensory disconnection, profound superficial analgesia, and moderate visceral analgesia, while protective airway reflexes (swallowing, coughing, pharyngeal/laryngeal reflexes) remain intact. The eyes stay open. The animal may show athetoid movements (slow, involuntary limb movement) — this is normal and must not be interpreted as inadequate anaesthesia depth. Critically: tiletamine alone causes poor muscle relaxation and can trigger convulsions.

Zolazepam is a benzodiazepine that addresses exactly those weaknesses. It adds anxiolysis, muscle relaxation, and anticonvulsant activity — preventing the tiletamine-induced convulsions and significantly improving muscle relaxation quality. It also smooths and accelerates recovery from anaesthesia.

The synergy between the two is what makes Zoletil work: tiletamine provides the anaesthetic depth and analgesia, zolazepam provides the safety net (anticonvulsant), muscle relaxation, and recovery quality.

Species Differences in Zoletil Pharmacokinetics

There is a critical pharmacokinetic difference between cats and dogs that affects recovery quality:

• In cats: tiletamine is eliminated faster than zolazepam — cats recover with residual zolazepam sedation, producing a smooth, calm recovery. Cats also generally tolerate Zoletil well and can receive higher doses safely.
• In dogs: zolazepam is eliminated faster than tiletamine — dogs recover with residual tiletamine effect and less benzodiazepine coverage, which can produce a rougher, more dysphoric recovery with potential for athetoid movement and vocalisation.

Dosage Chart for Dogs and Cats

Top-up dosing: If the procedure extends beyond the initial anaesthesia, re-dose at 1/3 to 1/2 of the initial dose. The maximum cumulative dose (initial + all top-ups) must never exceed 30 mg/kg in dogs or 72 mg/kg in cats.

Atropine Premedication

Hypersalivation is the most common side effect of Zoletil. It is controlled by administering atropine sulfate subcutaneously 15 minutes before Zoletil:

• Dogs: 0.1 mg/kg SC, 15 minutes before Zoletil
• Cats: 0.05 mg/kg SC, 15 minutes before Zoletil

How to Reconstitute Zoletil

1. Remove flip-off caps from both the powder vial and the sterile water diluent vial
2. Clean both rubber stoppers with an alcohol swab
3. Draw up all 5ml of sterile water into a syringe
4. Inject the 5ml into the powder vial
5. Gently swirl until completely dissolved — do not shake vigorously
6. The solution should be clear to slightly yellow; discard if cloudy or particulate
7. Label the vial with the date and time of reconstitution

Shelf life after reconstitution: 8 days refrigerated at 4°C in a dark location, or 24 hours at room temperature. Discard unused solution after these periods.

Critical Drug Interactions — Read Before Use

⚠️ Acepromazine (Phenothiazines) — Contraindicated

This is the most important interaction to know. Combining Zoletil with acepromazine or other phenothiazine-derivative drugs causes severe respiratory depression, myocardial (cardiac) depression, hypotension, and hypothermia. Pulmonary oedema has been reported in cats with this combination. Deaths have occurred. Do not use acepromazine as a premedicant before Zoletil. Use atropine instead for salivation control.

Chloramphenicol — Prolonged Anaesthesia in Cats

Concurrent use of chloramphenicol with Zoletil prolongs anaesthesia in cats by approximately 30 minutes by inhibiting hepatic metabolism. Avoid chloramphenicol during the pre- and intraoperative period in cats.

Barbiturates and Inhalant Anaesthetics

Both classes produce additive CNS depression with Zoletil. If used alongside Zoletil (e.g. Zoletil induction followed by isoflurane maintenance), significantly reduce the dose of the inhalant or barbiturate to avoid anaesthetic overdose.

Contraindications

• Severe cardiac or pulmonary dysfunction
• Pancreatic disease
• Pregnancy (any stage) and Caesarean section
• Feline renal insufficiency (tiletamine is renally excreted — impaired clearance causes dangerously prolonged anaesthesia)
• Hypertension
• Hypersensitivity to tiletamine or zolazepam

Recovery Management

• Place the animal in a quiet, dimly lit, warm recovery area
• Keep on a padded surface — athetoid movements may cause minor self-trauma on hard surfaces
• Monitor respiratory rate, heart rate, and body temperature throughout recovery
• Cats and small dogs with high body surface-to-mass ratios lose heat rapidly — supplemental heat (warm blanket, heat pad on low) is recommended
• Do not attempt additional doses to stop athetoid movements — this will cause overdose
• Recovery to full normal function: 2–6 hours depending on dose and individual animal

Overdose — Signs and Treatment

Early signs: loss of cranial and spinal reflexes, respiratory depression, tachycardia, cardiovascular depression. At doses above 20 mg/kg: significant CNS and cardiorespiratory depression.

Treatment is primarily supportive: secure and maintain a patent airway, provide assisted ventilation with oxygen, monitor vitals. Doxapram 5.5 mg/kg IV has been shown to antagonise the tiletamine-zolazepam combination, increasing both heart rate and respiratory rate and reducing arousal time. There is no complete specific reversal agent for tiletamine.

Where to Buy Zoletil

You can order Zoletil 100 and Zoletil 50 (Virbac) from PetShopBoss.com with free worldwide shipping. Both concentrations are available.

Also useful for anaesthetic protocols: Cerenia Injectable 20ml (anti-nausea) | Catosal 10% (metabolic recovery support)