A dear friend ran into big trouble at her 20-year high school reunion. She ran into an old flame, and the chance encounter reignited old sparks.

The problem? They were both married. There was no happily ever after — just lots of gut-wrenching drama and one painful divorce.

Nowadays, no one has to buy a flattering dress and travel to a forgotten hometown to reconnect with a former love. Virtual reunions take place daily all over the Internet. With a few clicks on Classmates.com or with a bit of searching on Facebook, you’ll stumble across the one who broke your heart or vice versa.

But think long and hard before sending that message or friend request. Nancy Kalish, a developmental psychology professor at California State University in Sacramento, has researched thousands of lost love reunions and has seen the disastrous aftermaths.

“These feelings can come back,” she warns. “And it will destroy you.”

She’s noticed that more people in their early 30s are becoming entangled in an affair that can become an online addiction. Many are married and have little children, she said.

Back when people had to physically track down a lost love, the process of reuniting was more difficult and more deliberate. Now, it’s casual. And the intensity of resurfaced feelings can catch some people off guard. It can be like a hit of cocaine for a recovering addict.

Kalish said some neuroscience research suggests that raging teenage hormones are stored as sensory and emotional memories. Early loves may be imprinted on the brain the way cocaine addiction is. When you see that person again or chat online, the reconnection can trigger visceral feelings of being young and in love.

“It’s happy marriages that are almost more at risk,” she said. In her surveyed group, 62 percent of rekindlers say they were married before they reconnected; about half report they had good or excellent marriages.

But once they’ve restarted the old relationship, they have trouble stopping, she said.

Kalish suggests the best way to handle a blast from the past is to politely respond: “It’s good to hear from you,” and offer some catch-up information. If there is a second attempt at communication, she says it’s best to gently cut it off.