Eric Cooper and his son Cooper Chen
Eric Cooper and his two-year-old son Cooper Chen who were allegedly murdered by Cooper's partner Dr Louis Chen  Globewriter/Flickr

Lawyers for a doctor who is alleged to have viciously stabbed his partner more than 100 times and repeatedly cut the throat of his two-and-a-half year old son may argue that cough syrup induced psychosis cause him to kill, it has emerged. Dr Louis Chen was naked and covered in blood at his Seattle apartment when he opened the door to his new boss who had turned up to enquire about why he had not turned up to work, according to court records obtained by The Seattle Times.

When a police officer asked the 43-year-old endocrinologist who had done it shortly after the gruesome discovery was made on August 11, 2011, he reportedly replied: "I did."

Chen had recently moved to the city from North Carolina with his partner Eric Cooper, 29, and their son Cooper Chen, so he could take up a job at a hospital. He was charged with two counts of first degree murder shortly after his arrest, but legal wrangling has seen him avoid trial for more than four years.

Chen's legal team had previously indicated it would pursue an insanity or diminished capacity defence. To claim insanity they would have to prove that Chen suffered from a mental disease or defect to the extent that he was unable to understand "the nature and quality of the act" or was unable to tell right from wrong, according to Washington state law.

With a diminished capacity defence however, voluntary intoxication can be considered when evaluating the defendant's state of mind.

Court records show that 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.

In a court motion filed in October his team argue that the drug metabolized slowly in Chen, who is Taiwanese, due to his genetic makeup. This, they contend caused him to suffer from psychosis before and after the killings, the motion says.

Chen goes to trial in April.