More than four years after Dr. Louis Chen was charged after the knife attack that killed his partner and toddler son, his defense team revealed in court documents that it intends to argue Chen’s use of over-the-counter cough syrup contributed to his psychosis.

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It’s been more than four years since Dr. Louis Chen was charged with aggravated murder in connection with the deaths of his longtime partner and their toddler son, but the first hint of his likely defense — that cough syrup contributed to his psychosis — only recently emerged during a legal debate over an expert witness.

Suspected prescription-drug abuse and a looming child-custody battle are described in search warrants as possible motives for the deaths of Eric Cooper, 29, and 2½ -year-old Cooper Chen, whose bodies were found inside their 17th-floor apartment in Seattle’s First Hill neighborhood in August 2011.

While Chen’s defense team announced nearly three years ago that it intended to pursue an insanity or diminished capacity defense at trial, the basis for that defense remained a mystery.

That is, until the defense filed a motion in October suggesting Chen was suffering from cough-syrup induced psychosis as a result of a buildup of the drug dextromethorphan in his system. The motion further intimates that the drug metabolized slowly in Chen, who is Taiwanese, due to his genetic makeup.

The revelations were made as the defense sought to bar an expert witness — a former state forensic scientist who resigned under a cloud as the head of the State Patrol Crime Lab in Seattle — from testifying when Chen, now 43, is tried on two counts of aggravated first-degree murder in April.

King County Prosecuting Attorney Dan Satterberg decided early on he would not seek the death penalty against Chen, leaving life in prison without the possibility of release as the only possible punishment should Chen be found guilty of aggravated murder.

It wasn’t immediately clear if cough-syrup-induced psychosis had ever been used as the basis of a defense in a murder trial in the state.

“I’ve definitely heard of induced psychosis through taking various drugs,” but never a criminal case involving cough syrup, said University of Washington law professor Mary Fan.

She said cough syrup can produce the equivalent of a high, and like any drug that’s misused or abused it can lead to intoxication.

“It’s really going to come down to, is it voluntary or involuntary (intoxication)?” she said. “That’ll be a big issue.”

Recent move

Chen, an endocrinologist, had recently moved to Seattle with his family from North Carolina for a job at Virginia Mason. When he didn’t show up for his first day of work on Aug. 11, 2011, a hospital manager went to Chen’s apartment, court records say.

When Chen opened the door he was naked and covered in dried blood. Police found the bodies of Cooper, who had been stabbed more than 100 times, and the couple’s son Cooper Chen, whose neck had been repeatedly cut, the records say.

An officer asked Chen, “who did this?” and Chen responded, “I did,” according to records.

Search warrants filed in the case indicate Chen was in the midst of an acrimonious breakup with Cooper.

Inside Chen’s apartment, homicide detectives found a variety of medications, along with prescriptions and blood-soaked notes written by Chen, which they say provided evidence of a possible dual motive: that Chen feared Cooper would report his prescription-drug abuse and the abuse of his prescribing powers as a doctor, or use that information against Chen in a future child-custody dispute.

Police also reviewed Chen’s finances since Chen had promised to pay Cooper $130,000 in the event their relationship ended, the warrants say.

An analysis of Chen’s blood showed he had dextromethorphan, a cough suppressant in many over-the-counter cold and cough medicines, as well as the anti-anxiety drug Xanax, and an antihistamine in his system.

Chen spent 10 days at Harborview Medical Center, where he was treated in intensive care for an overdose of medications and self-inflicted stab wounds, according to a heavily redacted mental-health evaluation by psychiatrists at a state psychiatric hospital. He reported hallucinations and had to be restrained for a few days because he had assaulted nurses while delirious.

While the Western State Hospital doctors could not provide a psychiatric diagnosis, largely due to a lack of information, they determined Chen showed no signs of psychosis or symptoms of a major mental illness at the time of his evaluation in December 2011.

They also noted Chen was only given antipsychotic medication by staff at the King County Jail after persistent, repeated requests from Chen and his defense attorneys.

Defense differences

Though insanity and diminished capacity are both mental defenses, they are conceptually different.

Insanity requires the defense to prove a defendant, at the time a crime was committed, suffered from a mental disease or defect to such an extent he or she was unable to understand “the nature and quality of the act” committed, or was unable to tell right from wrong, according to state law.

With an insanity defense, even if a defendant admits he or she intended to commit the crime, the act was done under a delusion.

Voluntary intoxication does not constitute insanity under the law.

With diminished capacity, however, any incapacitating factor — including voluntary intoxication — can be considered when evaluating a defendant’s state of mind. Basically, one’s mental condition was so diminished that he or she could not form the intent required to commit the crime.

In Chen’s case, mental-health evaluations conducted by defense experts are not part of the public record, so at this point there are only hints as to what may be presented at trial to support an insanity or diminished-capacity defense.

The defense contends Chen was suffering from psychosis in the days before and after the killings. That psychosis was caused, in part, by his use of an over-the-counter cough syrup containing dextromethorphan, says a declaration filed by attorney Todd Maybrown in October.

“The defense will also present evidence that Dr. Chen has a genetic makeup which compromises his ability to metabolize this chemical,” he wrote.

The argument appears to allude to scientific research showing a wide variability in how quickly people metabolize certain drugs based on production of a key enzyme. It appears that people of Asian descent may have a 50-50 chance of being born with a functioning gene variant for the normal metabolism of drugs like dextromethorphan. But roughly 40 percent of Asians and Pacific Islanders have reduced function, indicating a slower metabolism, according to a 2002 article in The Pharmacogentic Journal.

Slow or poor metabolism can increase the chances of adverse drug interactions or lead to a toxic buildup in the body.

Debate over testifying

Chen’s cough-syrup defense only came to light amid a larger debate over whether former state toxicologist Barry Logan would be allowed to testify against Chen.

Logan resigned as director of the Patrol’s Forensic Laboratory Services Bureau in 2008 after a panel of King County judges found he bore much of the blame for the lab’s sloppy work, ethical lapses and “fraudulent and scientifically unacceptable” practices in analyzing breath tests used to prosecute drunken drivers.

That same year, Logan went to work for a private laboratory in Pennsylvania.

Logan has authored or co-authored at least three articles in scientific journals about dextromethorphan abuse and the drug’s psychological effects, including psychosis, hallucinations and mania.

At the highest levels, dextromethorphan — found in products like Robitussin and Coricidin Cough & Cold — can lead to a dissociative state, resulting in violent, unpredictable behavior, likely without the intent of the individual, according to Logan.

The question of whether to allow Logan to take the stand had little to do with his actual research, instead centering on the issue of fairness since Logan had consulted with the defense and then two years later was retained by the state.

In June 2013, Maybrown, the defense attorney, discussed Logan’s research in connection with Chen’s criminal case, according to court records and court arguments. Maybrown ultimately decided not to retain Logan as a defense expert due to concerns about Logan’s “professionalism and business practices,” the records say.

But two mental-health experts hired by the defense referenced Logan’s journal articles about dextromethorphan in opining that Chen was psychotic, and therefore incapable of premeditating or intending to kill the alleged victims, Senior Deputy Prosecutors Don Raz and Mary Barbosa wrote in their reply brief.

Raz had hired Logan after confirming Logan had not been retained by the defense — and included Logan’s name on the state’s preliminary witness list in September. That prompted Maybrown to file a motion seeking to disqualify Logan from testifying, arguing he had been privy to confidential information.

King County Superior Court Judge Bill Bowman ultimately ruled Logan will be allowed to testify as a rebuttal witness, but only if the defense’s mental-health experts take the stand before Logan does.