Rumours also circulated at the time that Miss Murphy was a cocaine addict and had an eating disorder.
Now pathologist Dr Richard Shepherd, who was involved in examining the deaths of Princess Diana and murdered Crimewatch host Jill Dando, has claimed Miss Murphy's life could have been saved if she had been seen by doctors 24 hours earlier.
Dr Shepherd said: "By the time Brittany collapsed in her bathroom there was probably nothing the paramedics or hospital staff could have done to save her. But had she been taken to hospital 24 hours earlier and given intravenous drugs, there is a very good chance she would have survived and would still be here today.'
The pathologist doesn't say it was deliberate, nor does he directly blame the mother. But it is the mother who called the ambulance in the end. Dr Shepherd does not blame Sharon for her daughter's death and in the documentary explains they may not have realised how ill she was as she was addicted to prescription drugs and was in a sedated state.
Brittany Murphy, right, rose to fame for her role alongside Alicia Silverstone and Stacey Dash in the movie Clueless
He said: "Brittany was so sedated it's quite possible neither Simon nor Sharon had any idea how ill she actually was. That may be the reason why they didn't call an ambulance."
In the days and weeks leading up to Miss Murphy's death she had been taking a "cocktail" of prescription medication, Dr Shepherd said, including anti-depressants, sedatives and anti-anxiety drugs.
She was also taking cough medicine to help ease the symptoms of her chest infection.
According to the pathologist the combination of drugs in Miss Murphy's system might have contributed to her death.
Dr Shepherd said: "Drugs can have a very beneficial effect but they also have side effects. And mixing them together means that these side effects can combine and become lethal."
He added: "Cough medication is designed to stop you coughing so that you feel better. But actually it also stops you bringing up the mucus and the bacteria out of your lungs. So it tends to make pneumonias worse. Brittany was perhaps coughing but she wasn't coughing enough or fiercely enough to clear the mucus in her lungs."
Theories about Miss Murphy's death have ranged from anorexia to cocaine and even suspicion she was poisoned - none of which Dr Shepherd's analysis of her autopsy report found to be true. Traces of Methamphetamine, for example, came from an inhaler not illegal drug usage.
But he said there is evidence the poor condition of her home - described by her mother-in-law Linda Monjack as "a place of unhealthiness" - exacerbated her condition.
At the time, Miss Murphy's mother asked the pathologist to investigate whether fungi in the mould growing on her walls and poorly ventilated rooms could have contributed to her death.
Dr Shepherd said: "The autopsy report shows they didn't find any fungi, either in her blood stream or in the sections of lung that they examined under the microscope. So mould and fungi haven't played a direct role in the death of Brittany.
"But living in poor housing conditions like that is likely to have had a debilitating effect and contributed to her infection and death." He added: "She was apparently living in appalling conditions."
Autopsy: The Final Hours of Brittany Murphy, which airs on Thursday in the UK, will also expose the extent of Miss Murphy's paranoid fear she was being watched.
As well as being fearful of the paparazzi, she and Mr Monjack, believed they were being targeted by the American Government, after Miss Murphy gave evidence in a whistle blowing trial involving a friend.
The couple, who were married in 2007, became reclusive within their home, where they installed 'Fort Knox' style security, including a fingerprint-recognition locking system, CCTV cameras.
And they hired a guard to patrol the property 24-hours a day.
Describing their set up as "completely raving mad", Mrs Monjack said: "They would say there were helicopters flying over-head, they were being watched, both of them were very fearful of this. The house was like Fort Knox. They had surveillance cameras all around the house. And they had a security guard there 24 hours. The security was mad. Totally completely raving mad. It was way over the top. And it was because they were frightened."
She added: "They were very reclusive."
- Daily Mail