An official cause of death has yet to be released but a close friend denied speculation that Irons, 32, had a drug problem.
"The guess is that he probably took a sleeping pill to get a good night's sleep and get on the plane the next morning," Randy Rarick told "Good Morning America."
But the police report noted that generic versions of the prescription drugs Ambien and Xanax were found in the Dallas hotel room where Irons was found dead Tuesday. And the Honolulu Star-Advertiser reported that methadone was found in one of the bottles, citing the medical examiner's office.
Irons was on his way from a surfing competition in Puerto Rico to see his pregnant wife in Hawaii when he died.
His family has maintained that Irons, whose first child is due next month, died after contracting dengue fever during a recent trip to Portugal.
"Coming into Puerto Rico, he was fired up and ready to compete," Rarick said, adding that his friend appeared to be in good physical shape and happily signed autographs shortly after he arrived.
But Irons quickly fell ill, Rarick said, and sat out of the first round of competition. He then dropped out of the competition before the second round and was put on intravenous fluids by a doctor for dehydration.
Rarick said the doctor in Puerto Rico had advised Irons to stay on the island for a few more days to get stronger, but that the three-time world champion was eager to get home to his wife because he was no longer competing.
He was feeling better, Rarick said, but became more ill on the plane from Puerto Rico to Dallas. He checked into the hotel room to get a good night's sleep before connecting to Hawaii.
Hotel workers found Irons after he failed to answer a wake-up call.
Irons had returned to the world of competitive surfing this year after taking a year off to deal with personal issues. But Rarick said reports that Irons had battled a serious drug addiction during his time off were "probably overblown."
The Irons' family has said that he told them he'd contracted dengue fever in Portugal. The virus, transmitted by mosquitoes, can cause shock, organ damage and death, if left untreated.
A preliminary autopsy showed no signs of foul play. The medical examiner's office in Texas told ABC News that a toxicology test will be conducted to look for drugs in his system. The results are expected in four to six weeks.
The medical examiner's office would not confirm the report from Honolulu that methadone was found in his room.
Irons admitted he was battling personal issues in the recent documentary, "I Surf Because."
"If I didn't have surfing to get those out of my system," he said, "I would self-destruct."
His death has shaken the surfing community he so loved.
"It is unfortunate and it's really been a shock to the whole world of surfing," Rarick said. "The word spread like wildfire and nobody could believe what happened."
To learn more about Dengue, including how to spot the symptoms, CLICK HERE to visit the Centers for Disease Control and Prevention Fact Sheet.
If you think you or a family member could have dengue, consult a doctor immediately. If the doctor said the patient does not need to stay in the hospital, there are many things you can do to help him or her recover. Below is a list of things to do compiled by the CDC.
WHILE THEY HAVE A FEVER:
Let the patient rest as much as possible.
Control the Fever
Give acetaminophen or paracetamol (Tylenol) every 6 hours (maximum 4 doses per day). Do not give ibuprofen (Motrin, Advil) aspirin, or aspirin containing drugs.
Sponge patient's skin with cool water if fever stays high.
Dehydration can occur when a person loses too much fluid from high fevers, vomiting, or poor oral intake.
Give plenty of fluids and watch for signs of dehydration.
Bring patient to clinic or emergency room if any of the following signs develop: decrease in urination, few or no tears when child cries, dry mouth, tongue or lips, sunken eyes, listlessness or overly agitated or confused, fast heart beat (more than 100 per minute), cold or clammy fingers and toes, sunken fontanel in infant
Prevent Spread of Dengue in Your House
Place patient under bed net or use insect repellent on the patient while they have a fever. Mosquitoes that bite the patient can go on to bite and infect others.
Kill all mosquitoes in house and empty containers that carry water on patio.
Put screens on windows and doors to prevent mosquitoes from coming into house.
WHILE FEVER IS GOING AWAY:
Watch for warning signs as temperature declines 3 to 7 days after symptoms began. Return IMMEDIATELY to clinic or emergency department if any of the following warning signs appear:
Severe abdominal pain or persistent vomiting
Red spots or patches on the skin
Bleeding from nose or gums
Black, tarry stools
Drowsiness or irritability
Pale, cold, or clammy skin