Sedation of a senior or sick animal with an opioid and midazolam
One IM injection, both drugs in one syringe; sedation with little cardiovascular effect for senior, sick or debilitated animals, or premedication before induction. Not suitable for young, healthy animals.1,2,3
This page is an educational reference for veterinarians and veterinary students. It does not replace a thorough examination, clinical judgment, or the attending veterinarian's final decision. Doses are for adult animals, per kilogram; the amount and volume of each drug for a patient, and the doses for neonates and seniors, are calculated in the tool itself.
Step by step
per kilogram of body weight
1 Preanesthetic preparation Before the injection
Healthy animal: allow free access to water; withhold food for 4–6 hours before anesthesia.4
Animal under 8 weeks or under 2 kg: withhold food for no more than 1–2 hours, monitor blood glucose before, during and after induction, and schedule it as the first patient of the day.4
Animal with a history or risk of regurgitation: withhold food and water for 6–12 hours.4
Route: Intramuscular (IM) or slow intravenous (IV)
This combination is for senior, sick or debilitated animals: in young, healthy animals it usually does not sedate well and may cause excitement.1,3
Give it with the opioid in the same syringe, as a single IM injection.1
In a sick or debilitated animal, give the low end of the range.1
If an IV catheter is in place, both drugs can be given at the same dose, in the same syringe, slowly IV; by the IV route, give the low end of the range. IM absorption of midazolam is variable.1,2,5
Effects of this drug
On its own it produces little sedation; with an opioid, it sedates senior or debilitated animals better.1
In young, healthy animals it may cause excitement and restlessness instead of calming.1,3,5
It is metabolized in the liver, and its effect is reversed by flumazenil.1,5
4 Onset From the injection
Watch the animal closely in a quiet place; sedation needs monitoring and support just as anesthesia does. Monitor breathing, heart rate and mucous membrane color.2
Give oxygen by mask to a deeply sedated animal; airway control may become necessary, so have intubation equipment ready.2
Methadone and morphine start to work 10–20 minutes after IM injection and 1–5 minutes after IV injection; buprenorphine after 20–45 minutes.2
The response to IM midazolam varies between animals; the effect may be less than expected.5
If this combination is the premedication for anesthesia: after IM injection, induce 15–30 minutes later; after IV injection, induce once it has taken effect: methadone and morphine 1–5 minutes, buprenorphine 20–45 minutes.1,2
5 Sedation About 40 minutes
Sedation with this combination lasts about 40 minutes; the opioid's effect lasts longer.1,2
If sedation is not enough for the procedure or the procedure is painful, complete the anesthesia with an IV induction agent, slowly, to effect; a sick or debilitated animal needs a lower dose than a healthy one.2
6 Recovery and reversal After the procedure
Continue monitoring and supporting the animal until full recovery, and maintain body temperature.2
Both drugs are reversible. If sedation is excessive or prolonged, or breathing is weak, reverse midazolam with flumazenil: 0.01–0.2 mg/kg IV, starting with the lowest amount and repeating as needed until effect.1,3
Methadone and morphine are reversed by naloxone: 0.002–0.02 mg/kg IV or IM; the lowest amount for partial reversal and the highest for full reversal. As the opioid is reversed, its analgesia also decreases or disappears; reassess pain.1,3
The percentage on each side is not a measurement but a qualitative grade from the sources: full 100, good 75, partial 50, low 25, none zero. It refers to the peak effect after the injection, not the whole duration.
HypnosisLoss of consciousness — 0% (None)
This protocol sedates rather than anesthetizes: it gives light to moderate sedation, and the procedure is done without the animal losing consciousness.2,3
If anesthesia is needed, an IV induction agent is added.1
In senior, sick or critically ill animals, opioids produce sedation, not anesthesia.7
The opioid is the only analgesic in this combination; for a painful procedure, also use local anesthesia wherever possible.2
NSAIDs are contraindicated in dehydration, hypovolemia, hypotension, liver disease, uncontrolled gastrointestinal disease and coagulation disorders.7
Muscle relaxationRelaxed muscle tone — 50% (Partial)
Midazolam relaxes muscle; the animal is calm but not anesthetized.1,2
Blood pressure: Little effect
At sedative doses, midazolam and the opioid have little effect on blood pressure; the opioid may lower the heart rate slightly.2,6
Midazolam – Little effect: Has little effect on the heart and circulation.6
Cautions and notes
Caution This protocol is for senior, sick or debilitated animals. In a young, healthy animal it usually does not sedate well and may cause excitement; use another protocol for it.1,3
Caution Not suitable for calming fractious or aggressive animals.1
Caution The opioid may depress breathing and slow the heart rate, especially in a sick animal; monitor breathing and heart rate and have oxygen ready.2
Caution Midazolam is metabolized in the liver; in liver disease, give the low end of the range and monitor the animal more closely.1,5
Reversal and emergency drugs
FlumazenilReverses midazolam
Ampoule 0.5 mg in 5 mL (0.1 mg/mL)
First dose, starting from the lowest amount — Intravenous (IV): 0.01 mg/kg1
If sedation is excessive or prolonged, or breathing is weak. If needed, repeat the same amount until effect; up to 0.2 mg/kg in total.1,3
The lowest amount is for partial reversal and the highest for full reversal. As the opioid is reversed, its analgesia also decreases or disappears; reassess pain.1,3
AtropineBradycardia
Ampoule 0.5 mg in 1 mL; do not confuse it with the 10 mg/mL ampoule