The painkiller Demerol can be summed up in just a few words: Highly addictive with a slew of side effects.

It’s also the same drug that is rumored to have been given to Michael Jackson in the moments before he “collapsed and stopped breathing,” according to a report in the British newspaper The Sun.

“Demerol has a tremendous amount of side effects,” Dr. Patrick Annello, an anesthesiologist and pain management specialist at St. Francis Hospital on Long Island told FOXNews.com. “It can cause rapid heart rate, arrhythmias — and given in high enough doses — it can cause respiratory depression or shallow breathing.”

That is exactly what a source told The Sun happened shortly after Jackson was administered an injection of the drug.

“After taking the Demerol, he started to experience slow, shallow breathing,” the source told the newspaper. “His breathing gradually got slower and slower until it stopped.”

There are also reports the icon was taking a cocktail of prescription drugs including antidepressants.

If antidepressants are accidentally given in conjunction with Demerol, it can cause a very bad hypertensive reaction, and the patient can have a heart attack and stroke as a result, Anello said.

Dangerous reactions can occur if Demerol is administered while a patient is taking other narcotic pain medications, sedatives, tranquilizers, muscle relaxers, or other medications that can make them sleepy or slow respiratory function.

Brian Oxman, Jackson family attorney, confirmed Jackson's use of other medications when he told CNN Thursday that they had gotten in the way of doing rehearsals.

"His injuries, which he had sustained performing, where he had broken a vertebra and he had broken his leg from a fall on the stage, were getting in the way,” Oxman said. “I do not know the extent of the medications that he was taking.”

If he took other medications that are sedating – which pain medications usually are – in conjunction with Demerol, it can definitely cause shallow breathing and decreased oxygen levels, ultimately leading to respiratory arrest or heart attack, Anello said.

What is Demerol?

Demerol is a narcotic analgesic with effects similar to morphine. It’s prescribed for relief of moderate to severe pain, and is sometimes used prior to- or during a surgical procedure to enhance the effects of the anesthesia.

“Demerol is an old drug. It’s been around for some time, but the potency of the drug – pain management-wise — is extremely low,” Annello said.

Unlike some common pain relievers, which block the nerve endings from transmitting messages of pain to the brain, Demerol acts on the central nervous system, basically tricking the brain by replacing the feeling of pain with a “high.”

“Why people think they like it when they take it, is that it provides euphoria — basically makes the patient not care about their pain anymore,” Anello said. “Essentially they get high off the medication.”

People who take Demerol for chronic pain often become addicted to the drug and require more and more of it to get the same effect.

Demerol is being taken off hospital formularies across the board because it’s not a very good pain medication, Anello said.

Some side effects associated with Demerol include:

— Lightheadedness;

— Sedation;

— Nausea/vomiting;

— Sweating;

— Confusion/hallucinations;

— Irritability/anxiety;

— Tremors;

— Depression;

— Seizures;

— Loss of respiratory function;

— Rapid heart rate/arrhythmias.

In the meantime, the world is awaiting autopsy results on the singer, although the Los Angeles County Coroners office said it could take weeks to complete toxicology test that would show exactly what was in Jackson’s system.

“When it comes to testing… the good part about Demerol is that it hangs around in the blood a long time,” Annello said. “So when they check the toxicology levels… it will hopefully be reflective of what Jackson received in the last few days. And if he was given Demerol, the results will clearly show that.”