Methadone Hydrochloride 10mg/mL
Methadone hydrochloride has the following structural formula:
Methadone is a potent morphine analogue neuroleptanalgesic which raises the pain threshold by binding to μ-opiate receptors. Methadone is pharmacologically similar to, but has three times the analgesic potency of morphine.
Methadone depresses the respiratory centre of the medulla and stimulates the vagal centre, resulting in reduced respiratory rate and minute volume, and bradycardia. Intestinal tonus is increased. All such effects are slight at clinical dose levels and may be prevented by prior administration of atropine. Convulsions and respiratory depression are seen at ten times the therapeutic dose.
Methadone is detectable in plasma within 10 minutes of subcutaneous injection. Firm protein binding appears to occur in various tissues, including the brain. Methadone undergoes extensive liver biotransformation and metabolites are predominately excreted in the bile and urine, together with small amounts of unchanged drug.
The narcotic action of methadone is antagonised by naloxone.
Methadone is administered for relief of pain in horses, dogs and cats. Analgesia peaks between 10 and 20 minutes post-administration and persists three to five hours.
In combination with acetylpromazine injection, methadone is an effective agent of restraint and analgesia in horses, facilitating treatment of wounds, handling and loading of difficult animals, suturing, laryngoscopy, castration and minor surgical procedures. Following intravenous administration onset of action is rapid and clinical effect is usually sufficient within 3 minutes to begin the procedure. Horses rarely become recumbent or ataxic after methadone/acetylpromazine administration, even at high doses. Clinical doses produce a somnolent but aware state in which the patient is tractable, placid and sufficiently coordinated to minimise risk of injury to animal or operator.
Methadone may be employed as a premedicant to xylazine sedation or barbiturate or ketamine anaesthesia to provide effective analgesia and restraint during painful procedures.
In dogs, methadone is used as a premedication for barbiturate anaesthesia. Barbiturates have extremely poor analgesic properties, even at high dose rates. Methadone premedication of barbiturate anaesthesia smooths induction and greatly minimises the risk of drug toxicity by enabling the dose of barbiturate to be halved.
In cats, methadone is used as a premedicant for inhalational anaesthesia. Compatability with acepromazine and propofol has been demonstrated. Repeat doses of methadone for analgesia can be given at 3 time points with 5 hour intervals.
For analgesia and restraint in horses, and analgesia and anaesthetic premedication in dogs and cats.
DIRECTIONS FOR USE
This product is contraindicated for use during parturition.
This product is contraindicated for use in animals with severe respiratory depression.
This product is contraindicated for use in pregnant, breeding or lactating cats, as safe use of the product in these animals has not been evaluated.
Safe use of this formulation in cats less than one year of age has not been established.
Published information supports the safe and effective use of methadone in cats older than 6 months of age.
USE with caution in animals with hypothyroidism, adrenocortical insufficiency, impaired renal and hepatic function, head trauma and shock.
DOSAGE AND ADMINISTRATION
Use the contents within 90 days of first broaching the vial. Discard the unused portion.
MEAT WITHHOLDING PERIOD (HORSES):
NOT TO BE USED in horses intended for human consumption.
If poisoning occurs contact a doctor or Poisons Information Centre. Phone Australia 131 126.
Return any unused product to pharmacy or pharmacy inspector.
Store below 30°C (room temperature). Protect from light.