Propofol - it is the drug at the centre of the Dr Conrad Murray trial, the substance that a coroner's report said killed pop star Michael Jackson.
It is a powerful anaesthetic, used in most operations that require patients to be unconscious.
The white liquid is known in medical circles as "milk of amnesia", and Michael Jackson referred to it as his "milk".
Dr Murray's lawyers are arguing the singer self-administered a fatally large dose.
But the prosecution says Dr Murray gave Jackson the propofol, as a sleep aid, and then failed to properly monitor the pop star.
The American Society for Anaesthesiologists said the drug is "meant only for use in a medical setting by professionals trained in the provision of general anaesthesia".
It said: "Though the drug is often used for procedures requiring sedation, patients can have extremely variable responses to the drug and some patients can become completely anaesthetised, including losing the ability to breathe."
A court in Los Angeles has heard testimony that Dr Murray was on the phone to another patient minutes before he discovered Jackson in difficulty.
Dr John Dombrowski is on the board of directors of the American Society of Anaesthesiologists and uses propofol regularly.
He told Sky News Online: "When you give anyone an anaesthetic, the whole point is to monitor them beat to beat, minute to minute.
"You never know if there is going to be a good or a bad outcome, which is why you must always be there."
The paramedics who attended the scene and bodyguards who were also there have testified that Jackson was not attached to any monitoring equipment.
Dr Dombrowski said, because the drug can impair a patient's ability to breathe, at a minimum he would monitor heart rate, blood pressure, respiratory rate and levels of oxygen in the blood.
He said: "If someone stops breathing because they have been given too much propofol, it is easy to fix... but you need to be right there with them."
He also said it was essential that any sedated patient be given supplemental oxygen.
"You just don't allow a patient under anaesthetic to breathe room air," Dr Dombrowski said.
Witnesses have said they did not see Jackson being given any oxygen.
The court has also heard that, on the day he died, Jackson had taken enough Lorazepam - a drug that treats anxiety and insomnia disorders - to put six people to sleep.
Dr Dombrowski explained that, properly used, propofol is extremely safe.
But it does not mix well with drugs in the benzodiazepine family of drugs, because it greatly diminishes a patient's ability to breathe and increases the risk of heart failure.
The doctor said: "I compare it to standing on thin ice and then jumping up and down on it."
The LA court has heard from the defence team that Jackson was a desperate drug addict - and his dependency on propofol was what led him to self-administer the anaesthetic.
Psychiatrist Dr Omar Manejwala is an expert in addiction medicine and has treated approximately 30 propofol addicts over the past eight years.
He said it is a "significantly addictive substance", and that some of his patients come to rely on it because of a "desperate desire to check out of reality".
He described some of the addicts as having bruises on their foreheads because it is such a fast-acting drug that people pass out and bump their heads after injecting it.
Dr Manejwala said: "People develop a tolerance to it, so they need to take more to achieve the same effect, they can't control their usage, and they often end up sacrificing things that are important to them as a result of their addiction.
"Some become addicted to the sense of euphoria that can be experienced when emerging from propofol sedation."