Surprisingly, the sedatives, anaesthetics and medicines for euthanasia are very similar for horses, ponies, mules and donkeys, and they have similar doses. The only exception is etorphine hydrochloride, which is a very potent analgesic with the ability to provide short-term anaesthesia. The use of this drug in mules or donkeys is hazardous. There are the alarming problems with recycling of the drug, and therefore its use in both donkeys and mules needs considerable planning.

Often sedatives, tranquillizers, analgesics and anticonvulsants are given in combination. I will describe these combinations and cross-reference them under each named drug. I will not include gaseous anaesthetic agents. Medicines for euthanasia will appear at the end of the chapter.

A long-established tranquillizer, this can be given either orally or by injection, the preferred route by injection being i/v. The results from subcutaneous (s/c) or i/m injection are less reliable. The drug is available in varying strengths, so the bottle should be checked carefully for the strength before calculating the dosage. It is not a good drug for restraint in the horse as the animal is soon alarmed. Noise should be moderated, as loud noise definitely stimulates the animal. If given by mouth 45 min before sedation is required, it can help to allow i/v administration of a more potent sedative. It is favoured for premedication by anaesthetists – a large survey indicated that anaesthetic mortality was reduced by its use. However, that has not been a universal experience. It is used by some clinicians to lower the blood pressure in cases of laminitis. However, there is little evidence-based medicine (EBM) to back up its use; indeed, some authorities claim it is contra-indicated.

An extremely potent analgesic that should be given by injection, the preferred route being i/v. However, it can also be given s/c or i/m. When given on its own it should be given at the highest dosage rate, i.e. 0.2 mg/kg. This will give immediate relief from visceral pain, but that only lasts for 45 min. There are two unwanted side effects at this dosage. First, there may be severe ataxia; secondly, there is a sharp rise in central nervous system (CNS) pressure, so it should not be given in cases of head trauma. On recovery ‘box-walking’ has been noted, but this is rare. Its principal use is in combination with an alpha-2 agonist to give reliable sedation. The principal alpha-2 agonists are detomidine, romifidine and xylazine. The dosages for these combinations are given under the respective drugs, below.

In certain countries – but not the UK – this drug has classified status and so special storage and usage rules apply.

A long-established drug, chloral hydrate is a hypnotic rather than a sedative. It is supplied as either a crystalline powder or a very concentrated solution; the latter should be diluted to form a 10% solution. Reportedly, it can be given orally by stomach tube. This must be painful as it is irritant to mucous membranes, but this effect will be masked by the hypnotic effect; there is no good reason for using the drug in this manner. It has two other uses. The first is in combination with thiopentone sodium (see below) to provide anaesthesia, and the second is when it is added to the drinking water to apprehend a very fractious horse. Since oral chloral hydrate is so irritant and bitter, all water needs to be withdrawn for at least 24 h before administration. This, I feel, creates a welfare issue and hopefully an alternative method can be found.

After premedication with acetylpromazine, good anaesthesia can be obtained by giving a 10% solution i/v at the rate of 1 g/100 kg (i.e. a 500 kg horse will be given 5 g). The horse will then be very sleepy and ataxic. A bolus of thiopentone sodium at the rate of 0.3 g/100 kg (i.e. a 500 kg horse will be given 1.5 g) will cause recumbency and anaesthesia. This can be maintained by further boluses of 0.2 g/100 kg of thiopentone sodium. The danger with this regime is that both chloral hydrate and thiopentone sodium are extremely irritant if given perivascularly.