MARGARET River grandfather Peter Miles had started taking antidepressant medication just weeks before he shot dead his wife Cynda, daughter Katrina and four grandchildren on their hobby farm.

Mr Miles had put on weight in recent months, which can be caused by antidepressants, and some close friends now suspect the medication he had been prescribed may have played a part in triggering or heightening his homicidal and suicidal thoughts.

As police continue their investigation into the murder-suicide early last Friday morning that left seven dead and shocked the nation, family and friends of the much-loved Miles’ are still grappling for answers about why the “knockabout, down to earth” farmer killed his family, then rang triple zero and turned the gun on himself.

Organic farmer and family friend Bee Winfield confirmed 61-year-old Mr Miles, who had been looking for work on Gumtree just two days before shooting dead his family, had been suffering depression and had “gone to doctors for help”.

“It seems the antidepressants he had been prescribed were not working,” she said, adding that some antidepressant medication had “no warnings on the box” but came with the risk of “terrible side effects” including suicidal thoughts or thoughts of harming others.

Camera Icon Bee Winfield. Credit: Daniel Wilkins

Another close friend, who visited the family just days before the shooting, said: “Cynda told us Peter had gone onto antidepressants in the last few weeks. I feel for the sake of society that these mind-altering drugs should be exposed as dangerous.”

Neighbour Felicity Haynes said she knew Mr Miles had been “depressed for some time” after losing one son to suicide over a decade ago and with another son needing an organ transplant, while there are also suggestions he was under financial stress and struggling to find farm maintenance work.

“(Cynda) said to me, ‘There are a few difficulties at home.’ Peter was becoming less rational and that was a worry to her,” Ms Haynes said.

The Australian newspaper yesterday reported Ms Miles told friend Cath Miller the day before the mass shooting that her husband’s depression was getting “worse and worse”.

“Peter seems to be getting worse and worse so I don’t know that he is going to be able to do the yeomans plough — can you find someone else? Really sorry to let you down but I thought better to let you know than hand on hoping he improves,” Ms Miles wrote in a Facebook message to Ms Miller.

The following morning, as news spread of the deaths, Ms Miller messaged Ms Miles: “Just checking to see if you are ok.”

Other family friends who spoke to The Sunday Times about Mr Miles’ medication stressed they did not know which type of antidepressant he had been prescribed. But they have genuine fears it may have belonged to a common class of the drug known as “selective serotonin reuptake inhibitors”, or SSRIs.

SSRIs boost serotonin levels in the brain and are considered effective in the vast majority of cases, but there are claims that in rare cases they can contribute to extreme violence, murder and suicide, particularly in the first few weeks they are taken.

In a 2004 court case a WA woman, who plead guilty to attempted murder after trying to kill herself and her two-year-old and nine-year-old daughters by gassing them in the family car, also blamed antidepressants prescribed to her. The court was told the mother, whose name was suppressed, would not have committed the offences if she hadn’t been prescribed high doses of SSRIs.

The Royal Australian and New Zealand College of Psychiatrists referred The Sunday Times to its guidelines, which state some patients taking SSRIs experienced an “immediate and distressing level and agitation” and the college’s clinical guidelines warn medicos to “monitor all patients for the emergence or worsening of suicidal thoughts during the first two to four weeks of treatment”.

Australia’s Therapeutic Goods Administration (TGA) warned that patients are often not aware of the risk of suicidal thoughts and behaviours when they begin treatment with antidepressants, and it urged doctors to effectively communicate the risks, particularly with SSRIs.

The TGA confirmed “treatment with antidepressants has been linked with a small increase in the risk of suicidal thinking and behaviour”, but also said it was difficult to determine whether it was caused by the drug or a symptom of the underlying condition.

In the US, the Food and Drug Administration requires the antidepressants carry a “black-box warning” due to the risk of suicidal thoughts or behaviours in children, teenagers and young adults. In the UK, a 2017 investigation by the BBC – which sparked a backlash from the medical community – claimed that SSRIs had been associated with 28 reports of murder referred to the UK medicines regulator in the last three decades.

Murdoch University experts Dr Bob Mead, an Emeritus Professor of Biochemistry, and Dr Ian Mullaney, a senior lecturer in pharmacology, said SSRIs were the most commonly prescribed antidepressants and can also be used to treat anxiety and obsessive compulsive disorders.

But in a joint statement, they said: “Whilst there is some evidence to suggest that these drugs during the first few weeks of use or after a change in dose are associated with an increased risk of suicidal thinking and behaviour in children and adolescents, there is no compelling evidence to suggest these drug-induced behaviours occur in adults over the age of 25 years.”

The Australian Medical Association was approached for comment.

WA Police would not confirm if Mr Miles was taking medication or what type of antidepressants he was on, although Police Commissioner Chris Dawson said officers would look at “all aspects” of what may have prompted the murders of Mrs Miles, 58, the couple’s daughter Katrina, 35, and grandchildren Taye, 13, Rylan, 12, Arye, 10, and Kayden, 8, at their farm in Osmington, east of Margaret River.

For Ms Winfield, the SSRI theory, while not proven, is the most plausible. “Nothing else explains it,” she said. “This was a lovely man whose family was everything to him. I don’t know when the depression set in, but I’m really sad it hit him so hard and I’m sure the medication was a part of it.”

The Miles’ surviving sons, Tom and Neil Miles, declined to comment.