The Vancouver coroner's office revealed Tuesday that “Glee” star Cory Monteith died from "mixed drug toxicity, involving heroin and alcohol."

The 31-year-old actor had been plagued by substance abuse problems long before his Hollywood days, and had voluntarily checked himself into an undisclosed rehab facility in March, completed a 30-day program, and checked out on April 28. And while it appeared to friends that Monteith was happy and on the road to recovery, experts we spoke to said addicts face serious risks in the weeks and months after they check out of rehab, especially if there is no follow-up care.

“One of the main reasons death can occur shortly after an individual is released from rehab is because of a large decrease in tolerance that their body experiences after a period of abstinence,” Dr. Adi Jaffe, co-founder and research director of Alternatives Addiction Treatment Center in Beverly Hills, told FOX411’s Pop Tarts column. “If they return to using, they might experience far more extreme reactions to drug doses they were once accustomed to using.”

Another Hollywood insider who has worked with many recovering drug addicts concurred that rehab can sometimes provide a “false sense of security” in believing that everything is okay and they’re recovered. Many also pointed out that a 30-day program for a lifelong addict -- Monteith has said he has struggled with drugs since he was 13 -- might not be enough.

“I have often seen when someone goes to rehab and detoxes off of drugs, they may not spend enough time to get to the root cause of their addiction. This usually takes between 90 and 120 days. 30 days is usually not long enough,” addiction specialist Dr. Damon Raskin said. “If a user then relapses, he may return to using the same dose that they used prior to the relapse, and they may overdose due to losing the tolerance he had prior. This frequently leads to death.”

More On This...

At 13, Monteith says he skipped school to drink and smoke pot, and by 16 was deep in the trenches of drug problems. At 19, he entered rehab for the first time after his mother and friends staged an intervention, but went right back to drugs before being ordered to clean up his act or face jail time after stealing a significant sum of money from a family member, which spurred him into making strides in the right direction. He soon took on odd jobs, and then in 2009 hitting the jackpot by scoring a role on the hit FOX musical show.

It remains unclear precisely which treatment facility Monteith visited this year and the specifics of his program, and others connected to him were not aware if and what type of follow-up treatment he was receiving post-inpatient. But just a few days after being released from rehab this year, Monteith was pap-snapped vacationing in Mexico with girlfriend and "Glee" co-star Leah Michele.

"Sailing off on another adventure," Michele tweeted before leaving Vancouver May 4. "Putting down the electronics for the next few days. Gonna rest up mind body and soul! See you soon!"

Experts we spoke to said in most cases a vacation is a huge post-rehab mistake.

“There is this notion that when you are in an inpatient facility and young come out that you’re fine and don’t need any post follow-up discharge, but without structure you get sucked right back into the vacuum. Most people need to go directly from residential into a community-based program. Structure is so important, so it is usually inadvisable (to go on an overseas vacation),” said Steven Horen, CEO of Koved Care, a private psychiatric care provider located in New York and California. “And 30 days of treatment usually isn’t enough if there are any underlying psychiatric issues. If someone isn’t accurately diagnosed from outset they might not pursue right treatment.”

Dr. Raskin concurred that the post-rehab programs are of critical importance, and that departing for a vacation is typically “unhealthy for someone just beginning their recovery.”

Monteith spent the week prior to his death in his native British Columbia, staying at the Fairmont Pacific Rim hotel and socializing with friends. On the night that he died it was believed that he was hanging out with friends in his room, before hitting the town and returning alone in the early hours. According to some Twitter posts, on Thursday he also hit up a local East of Main café with other associates. However, photographs have surfaced of Monteith out with friends at a coffee shop drinking and chilling out just days before his death. According to TMZ, Monteith “knocked down three cold ones” but wasn’t acting drunk or disorderly.

“The investigation into this death by the BC Coroners Service is continuing, and no other details are available at this time,” read the report issued by authorities. “It should be noted that at this point there is no evidence to suggest Mr. Monteith's death was anything other than a most-tragic accident.”

Despite his demons with drugs and alcohol, multiple sources have said Monteith was as happy as can be with life, personally and professionally.

“Cory was just a great guy,” added a close friend. “He touched so many lives.”

A representative for Michele also released a statement on Tuesday, saying: “Lea is deeply grateful for all the love and support she’s received from family, friends and fans. Since Cory’s passing, Lea has been grieving alongside his family and making appropriate arrangements with them. They are supporting each other as they endure this profound loss together.”