DENVER (CBS4) – Drug usage will play a big role in the defense of a man accused of killing his wife.

Richard Kirk, 47, is accused of killing his wife while their three children were inside the home. He was officially charged with first degree murder on Friday.

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Kristine Kirk, left, and Richard Kirk, right (credit: CBS)

Kristine Kirk, left, and Richard Kirk, right (credit: CBS)

Kirk’s sister told CBS4 she thinks his use of pain killers and marijuana pushed him over the edge. Whether or not he had a history of using marijuana with prescription drugs will be a factor in his defense.

Court documents say it was a type of marijuana edible candy police believe Kirk ate the night he’s accused of shooting and killing his wife, Kristine Kirk. Minutes before her death she was on the phone with 911 saying Richard was hallucinating, had taken marijuana and possible prescription medication.

CBS4 legal analyst Karen Steinhauser says Kirk’s metal state the night of the shooting will play a big role in his defense.

“The defense may be looking at two specific defenses, one is voluntary intoxication and the other is involuntary intoxication,” said Steinhauser.

Voluntary intoxication, like drinking too much alcohol, could mitigate the “intent” to commit first-degree murder — the charge he now faces. Without intent, a lesser charge would be second-degree murder. But involuntary intoxication could mean two things.

“One is that you don’t know you’re ingesting something but the other is you know what you’re ingesting or you’re taking prescribed medication but it ends up having a result that wasn’t anticipated at all,” said Steinhauser.

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Richard Kirk’s sister says that her brother was a model father and what happened that night was an extraordinary event.

If a jury finds Kirk not guilty by involuntary intoxication, he will go free.

“It is a complete defense and the person is returned to society,” said Steinhauser.

Steinhauser said the involuntary intoxication defense has played a role in similar cases involving Ambien, a popular prescription sleep aid.

Kirk’s next court appearance will be on Monday.

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