TMZ claim to have obtained documents showing Williams was suffering from Lewy Body Dementia (LBD) before he died.
The website also states that sources connected to the Williams family say LBD was the "key factor" that drove the actor to kill himself.
According to the NHS, recurrent hallucinations are a common symptom with this form of dementia.
But what else do we know about the illness?
LBD affects over 100,000 people in the UK. The illness often accompanies Parkinson's disease - which Williams is known to have been suffering from before his death.
With LBD, clumps of abnormal protein - called Lewy bodies - from in the brain, causing loss of mental ability and the gradual death of brain cells.
Despite the high figures, many people are unaware of what LBD is, often meaning they fail to seek the correct medical help.
Blogging on HuffPost Healthy Living, author Judy Towne Jennings said when her husband started to show LBD symptoms, she struggled to find information on the illness.
"Unfortunately, there are still many medical professionals who are unfamiliar with the disease or tell patients that what they have is Parkinson's disease with dementia," she wrote.
"I encourage all spouses to seek answers. When we know what we are dealing with, we have the opportunity to create an adventure, rather than succumb to a disaster."
Visual and auditory hallucinations are a common symptom of LBD. Speaking to HuffPost UK Lifestyle, Dr Doug Brown, director of research at Alzheimer’s Society said: "Visions or delusions are often of people or animals, and are detailed and convincing."
"Everyone’s experience is different and it is important to remember what they are facing feels real to them at the time.
"In most cases, it is unhelpful to try to convince those having visions that there is nothing there, or that what they believe is untrue. Instead, family, friends and carers can offer reassurance that they are there to support the person.
"If you’re worried about any aspect of your memory or thinking, speak to your GP."
In addition to hallucinations, those suffering from LBD sometimes experience sleep disturbances, slowed or stiff movement, tremors and fainting.
They may also suddenly switch from a state of alertness to a state of drowsiness or staring into space.
While there is no cure for LBD and no drugs that can slow the illness, medication that can ease symptoms if available from GPs.
According to TMZ, Williams had been complaining about the way his medication "made him feel" to wife Susan Schneider before he died.
Williams, who died age 63, suffered from depression for most of his life. It is currently unclear how far Lewy Body Dementia contributed to his death.
For confidential advice, information and support on Lewy Body Dementia, call Alzheimer's Society's National Dementia Helpline on 0300 222 11 22 or email email@example.com.
If you've been affected by the issues in this article, please call the Samaritans on 08457 90 90 90.