Cough syrup has some strange side effects, such as blurred vision, shakiness, confusion, and nausea — but could it be the catalyst to murder? That may be the case for Dr. Louis Chen, who was accused of murdering his 29-year-old partner Eric Cooper and their 2-year-old son. His lawyers propose to argue their client was not in control of his actions because he was suffering from cough syrup-induced psychosis.

Flash back to 2011: When Chen didn’t show up for his first day at work in a Seattle hospital, police went to his apartment to search for him, and it is there they found him covered in blood alongside the bodies of his partner and their son. He had stabbed his partner more than 100 times and slit his toddler’s throat. Chen spent more than a week in the hospital being treated for self-inflicted stab wounds, and now at the age of 43 faces life in prison without parole.

But in a recent turn of events, Chen’s defense team filed papers in court that indicate he was suffering from psychosis caused by a buildup of the drug dextromethorphan, otherwise known as cough syrup. Chen’s lawyers had warned they would pursue an insanity plea or diminished capacity defense.

The motion they filed suggests because Chen is Taiwanese, his genetic makeup slowed the metabolism of cough syrup down. Because it processed through his system at a slower rate, its effects could have built up in the body and led to enhanced and extreme symptoms.

Is it a plausible defense or a far reach? Let’s turn to science for insight. According to the American Academy of Family Physicians, patients vary widely in their response to certain drugs because of their genes. Experts estimate that genetic factors account for 20 to 95 percent of patient variability in response to individual drugs. Drug response comes down to a combination of genetics and environment. Genes can influence how quickly the body metabolizes the drug, but there are also genes that specifically control gender, and, in Chen’s case, ethnicity.

But even if his genes were to blame, does a common, over-the-counter drug have the power to compel someone to kill their own family? The dextromethorphan in the cough syrup Chen claims to have taken prior to the murders is a suppressant. It works by affecting the part of the brain that causes a cough. However, if a person overdoses on the medication, it usually leads to mild effects, such as nausea, vomiting, constipation, drowsiness, dizziness, blurry vision, confusion, slowed breathing, nervousness, and shakiness. It is a rare defense case, and no one will know if it’s deemed plausible until Chen appears in trial April 2016.