Dr. Drew Says Hayden Panettiere May Have Hidden a Relapse

Dr. Drew says Hayden Panettiere may have had a real stretch of sobriety before relapsing ... and says those relapses can stay hidden from those around them.

The good doc joined "TMZ Live" Monday after we first reported that Hayden is suspected to have died from an oxycodone pill laced with fentanyl.

Drew says Hayden appeared to have gotten sober, but says relationships are one of the things that can knock someone off track even after a year or two of sobriety.

That came up as Brian Hickerson entered the conversation ... Charles noted Hayden and Brian had long been described as on-and-off, though we've learned they were actually together the whole time.

Drew says the medical system can also cause people to relapse ... so it's not always personal relationships.

He says someone can relapse on oxy and still look totally fine to the people around them ... because addicts often hide it out of shame.

Drew also says counterfeit pills remain a major danger because users have no idea how much fentanyl may be inside.

As for the dealer who allegedly supplied Hayden, Drew says there's blame there ... but he doesn't think the dealer simply showed up and caused the relapse out of nowhere.

He says, to his eye, Hayden had real sobriety before something pulled her back in.

Brian has not been charged with a crime, and the DEA appears to be focusing on her longtime drug dealer in Los Angeles ... trying to trace the pill back to them.