On 11 December 2015, a man bought a return train ticket to Manchester from Euston station, London. From Manchester he travelled on to Greenfield, Saddleworth and stopped at the Clarence pub, where he asked about how to get to the top of the hill. Despite warnings from the landlord about the late hour and poor weather conditions, the man set off anyway. He was spotted twice making his way up the hill.

The following morning the man was found dead on Saddleworth moor, near Dovestone reservoir. He was found by a passing cyclist lying on his back with his arms by his side. His pockets held £130 in cash and three train tickets. There were no items on the body, such as credit cards or driving licence, that could be used to name him. Despite the best efforts of the police, his identity remains a mystery. The circumstances of his death are also mysterious, but some light has recently been shed on this aspect of a puzzling case.

Preliminary toxicology results have identified traces of strychnine in the dead man’s body. Strychnine is a notorious poison, but outside of Agatha Christie novels it rarely makes an appearance in the modern world. In Christie’s day strychnine was frighteningly easy to get hold of: you could buy it over the counter in any local chemist’s shop. Strychnine was available as a rat poison or as a medicine. Little old ladies, like Mrs Inglethorp in Christie’s 1920 novel The Mysterious Affair at Styles, were prescribed strychnine tonics to increase their appetite and invigorate their nerves.

Strychnine certainly invigorates nerves, but not in a medically beneficial way and has therefore been removed from the British Pharmacopoeia. Strychnine interacts with chemical receptors on motor neurones, those that control movement. Electrical signals are sent along nerves but at the junctions between nerve cells, chemicals (neurotransmitters) are released to pass on the message. Two neurotransmitters are important in the case of strychnine: acetylcholine, which is the “go” message telling the adjoining nerve or muscle cell to fire, and glycine, the “stop” message, which tells the nerve not to fire. The balance of these two chemicals prevents false signals triggering nerves to fire and muscles to spasm. Strychnine attaches to glycine receptors and blocks the stop signals. The result is that the slightest stimulus can cause the nerve to signal or a muscle to contract.

An overdose of strychnine usually starts with twitching but progresses to horrific whole body convulsions as the nervous system careers out of control. The muscles on the back of the body are usually stronger than those on the front, so victims arch upwards with their arms clenched at their sides and only the heels and back of the head resting on the ground. There will be a period of convulsion followed by a period of calm before another bout of spasms begins. Most victims survive between two and five of these bouts before the muscles controlling their breathing become paralysed and they suffocate. During the whole process the victim is horribly and painfully aware of what is going on, as the nerves of the brain are also stimulated to give heightened perception. It is an agonising way to die.

Thankfully, strychnine rarely causes death. Since Christie wrote her poisoning plots the rules around its purchase have been tightened considerably. A paper published in 1979 explains that in the preceding 30 years there was on average one death per year in the UK due to strychnine poisoning; all but one were suicides. Most of the individuals who took strychnine had obtained it in the form of pesticide. Until recently strychnine was still used by licenced pest controllers to exterminate moles, but in 2006 it was banned from this final use because it causes the animals unnecessary suffering. It is now illegal to purchase strychnine in the UK.

Where does this leave the case of the mysterious man on Saddleworth moor? Whoever he was, his body appears to have contained an illegal lethal substance. A second postmortem being conducted this week will hopefully reveal more details. As well as establishing the cause of death police are still working to identify the man. Hopefully someone will recognise the images of him captured on CCTV at Euston station, or know him from the police description - white, aged between 65 and 75, of slim build, with a receding hairline with grey hair to the side and back. He had a large nose that may have been previously broken, and was clean shaven. He wore a brown heavy jacket, blue jumper, white long-sleeve shirt, blue corduroy trousers and black slip-on shoes.

What is really tragic about this case is in the three months since this man’s death no one appears to have missed him. Anyone with information should phone police on 0161 856 8972 or the independent charity Crimestoppers, anonymously, on 0800 555 111.

Dr Kathryn Harkup is a chemist and science communicator. Her book, A Is for Arsenic: The Poisons of Agatha Christie, is published by Bloomsbury.

• This article was amended on 17 March 2016. An earlier version contained a line in the final paragraph implying that the death in the Saddleworth moor case was suicide. The police investigation is ongoing and no conclusions have yet been reached.