SAN DIEGO -- A Washington state couple accused of giving their 6-month-old son a fatal combination of over-the-counter medications both pleaded not guilty Tuesday to a murder charge.

Thomas Boettger Sr. and Lorrie Boettger, both 38, are each being held on $1 million bail, pending a June 20 preliminary hearing.

The Boettgers were charged last month with felony child abuse likely to cause great bodily injury in the death last Oct. 25 of Thomas Boettger Jr., who died during a family trip to San Diego. The murder charge was added Tuesday.

A medical examiner listed the cause of death as acute doxylamine (Unisom) and diphendydramine (Sominex or Benadryl) intoxication and classified the death as a homicide.

Attorneys for the couple have said their clients loved the child and only gave him the over-the-counter medications to ease his suffering from colic, a cold and a lack of sleep.

They argued last week that their clients should be facing nothing more than involuntary manslaughter charges because they didn't intend to harm their son.

But Deputy District Attorney Matthew Greco said that the Boettgers had been giving the infant over-the-counter adult drugs "over a period of time."

According to a declaration in support of the arrest warrant, Thomas Boettger Sr. told a medical investigator that his son woke up about 5 a.m. on Oct. 25 crying heavily, and he gave him a dose of Zantac and a half-tablet of Sominex.

The father later gave another account, in which he said he gave his son crushed half-pills of Unison, Zantac and Benadryl via syringes and bottles, said Medical Investigator James Ramirez.

When contacted last month, Lorrie Boettger told police that she and her husband took turns administering medications to their son on the day he died.

The defendant said she looked up the medications on the Internet to learn how to dispense them.