A vet will administer an overdose of anaesthetic drugs. In most cases a sedative will also be given beforehand.
Shortly after the anaesthetic injection the horse will fall to the ground, where the vet will monitor its pulse until it has ceased. The heart will then stop but there may be some natural but upsetting involuntary muscle twitching or leg movement. A lethal injection can only be administered by a vet. The horse will tend to fall quite slowly and unpredictably but this can be controlled by an experienced operator. The vet may allow you to hold your horse’s head but some people find it extremely emotional so think carefully about who you think should be there.