Wecht said the drugs caused the cardiac dysrhythmia that led to Daniel's death. Wecht got the toxicology results from National Medical Services in Willow Grove, Pa., on Wednesday.
"In my opinion it's an accidental death," Wecht tells PEOPLE.
Wecht said that because Daniel was taking three drugs and not a large dose of a single drug, paired with the fact that he was celebrating a happy occasion with his mother, those factors point to a "tragic accidental drug death."
Asked why Daniel may have been taking methadone, which is sometimes used to help heroin addicts end their addiction, Wecht said the drug is "a legitimate prescription drug for pain relief" and that Daniel had "no known history of morphine addiction."
Anna Nicole Smith and her family hired Wecht privately to perform a second autopsy after Daniel's death.
Daniel Smith died in his mother's hospital room Sept. 10 while visiting her in the Bahamas three days after she gave birth to a baby girl named Dannielynn Hope.
Smith's longtime lawyer, Howard K. Stern, said Tuesday that he is the father of the little girl.
Meanwhile, photographer Larry Birkhead, who has said he dated Smith for about two years, claims on his Web site, "I maintain that I am the proud father, and look forward to a paternity test."
On Sept. 9, Daniel Smith arrived in Nassau to see his mother, with whom he'd always been extremely close. The family, including his newborn sister, spent time bonding in the hospital room that night. Daniel was seen tending to his mother around 6:20 a.m. on Sept. 10 – but at 9:38 a.m. that day Anna Nicole woke up to find her son unresponsive.
Doctors were immediately summoned and tried for 22 minutes to revive Daniel without success. He was pronounced dead at 10:05 a.m.
Days after Daniel's death Anna Nicole's Bahamian attorney, Michael Scott, described her as "overwhelmed" by grief. "She's trying to come to grips with the depths of her despair," he said.
Stern, who appeared on Larry King Live Tuesday, said Anna Nicole's baby daughter is "the one thing that is really keeping her going.
"Through it all, even with all the pain, she has been a great mom, a very attentive mom," he said.