An expert witness won't be able to tell the jury in the Christopher Vaughn murder case how migraine medicine taken by the accused killer's wife may have driven her to gun down her family and take her own life.

While Judge Daniel Rozak is barring that "speculative" testimony from a defense expert witness, he is allowing the jury to see the safety warning for the prescription drug Topamax.

According to the Topamax website, the drug "may cause suicidal thoughts or actions in a very small number of people, about 1 in 500."

Rozak made the ruling during a brief hearing Tuesday morning in Will County court.

Vaughn, 37, has been jailed since June 2007 on charges he murdered his wife and three children.

Vaughn, his wife Kimberly, 34, and children Blake, 8, Cassandra, 11, and Abigayle, 12 were heading from their Oswego home to a Springfield waterpark when he pulled off Interstate 55 and stopped on the frontage road outside Channahon.

Christopher Vaughn then murdered his wife and children as they sat in the family's minivan, according to police and prosecutors.

But Vaughn told detectives it was actually his wife who shot down their children, put a bullet in his leg as he fled for his life, then turned the gun on herself and committed suicide.

Christopher Vaughn reportedly explained to detectives that he had confessed to his wife about an extramarital dalliance, and that was what led to the unhinged Kimberly Vaughn's killing spree.

Vaughn also told investigators that the Topamax his wife was taking for migraine headaches contributed to her becoming both homicidal and suicidal, officials said.

Besides suicidal thoughts, the Topamax website says the drug may cause eye problems, decreased sweating and increased body temperature, soft bones, stunted growth in children, confusion, tingling in arms and legs, loss of appetite, nausea, taste change, diarrhea, weight loss, nervousness, and upper respiratory tract infection, among other things.

Vaughn's trial is set to start Aug. 13.