A bald man in gray sweats bounds into the brick plaza next to City Hall.

"Hey," someone calls out, "where you been?"

"At the hospital," the man named Anthony says. "I OD'd."

A half dozen people watching shake their heads. It's a bad week in Chelsea, they say, with three overdose deaths.

"They're dropping like flies," says Theresa, a woman who manages a rooming house and does not want to share her last name.

Anthony, whose last name we’ve also agreed not to use, says he overdosed the night before on a particularly strong bag of heroin, laced with fentanyl, the dealer said, or something like it.

"[The dealer] told me how strong it was," Anthony says, "but everyone says that to sell their dope."

Fentanyl, an opiate that is many times more powerful than heroin, was present in about 37 percent of overdose deaths from January through June of last year, based on 502 cases analyzed by the Office of the Chief Medical Examiner in Massachusetts.

What this chart from two Harvard researchers shows: "The most frequent drug category cited in overdose deaths was heroin (39.1%), followed by non-specified opioids (36.9%) and fentanyl (also 36.9%). Cocaine was cited in 23.4% of cases, ethanol (alcohol) 18.8%, and benzodiazepines 13.0%. “Chronic substance use," a vague description which did not refer to a specific drug class but infers that drug use was implicated in the death, was cited in 2.4% of cases. ... The vast majority of opioid-related overdose deaths in the first half of 2014 involved more than one drug."

The drugs implicated in overdose deaths in Massachusetts from January through June 2014, according to the Massachusetts Office of the Chief Medical Examiner (Chart courtesy of Vaughan Rees, PhD, and Christopher Knapp/Harvard T.H. Chan School of Public Health)

Including his latest, Anthony says he has overdosed 12 times. Most of his near-death experiences did not include fentanyl -- as far as he knows. The really intense and dangerous highs were produced by heroin, sometimes with an alcohol chaser, and pills.

"The potency now is so inconsistent, you don’t know what to expect," Anthony says. "So you’ll eat a bunch of Klonopins and do a shot of heroin and then you’re dead."

When a heroin drug combo doesn’t kill you, "it intensifies the heroin high, and keeps you high longer," Anthony says.

Klonpin, Xanax and other anti-anxiety medications are benzodiazepines, also known as benzos. They showed up in 13 percent of Massachusetts overdose death cases sampled last year. The vast majority of deaths were caused by heroin in combination with some other drug or alcohol.

Using heroin in combination with other drugs is certainly not new. The speedball mix of heroin and cocaine that is present in some of the overdose deaths has been a popular high-risk choice for decades. Patients on methadone describe a popular cocktail taken after their daily dose of methadone treatment: Gabapentin (anti-seizure med), Klonopin, Clonidine (treats high blood pressure) and an over-the-counter allergy medicine.

Some heroin users we spoke with for this story did not want to comment out of fear that highlighting the heroin combo issue would restrict the supply of some of these drugs.

"Narcan given" counts are narcotic-related incidents in which the patient’s condition required administration of Narcan, an overdose reversal drug. Boston Public Health Commission Interim Director Huy Hguyen says polydrug, or multi-drug, use is up in Boston. (Source: Boston Public Health Commission)

But as overdoses and overdose deaths rise, with the majority a result of multiple drugs, some doctors and patients are asking: What can we do?

Some heroin users say the answer is simple: Steer clear of combinations that increase the risk of an overdose.

"No combos for me," says a man who asks not to be identified. "I’ve seen a lot of my friends die."

This man had a scary experience with alcohol and benzos a few years ago. "When I mixed it with booze, that’s a bad combination for benzos. You know what I mean, you can end up off the bridge," he says.

But on the streets here in Chelsea, there’s a hot market for pills taken with heroin. These are drugs that slow respiration, the heart rate and produce other dangerous side effects.

"So you think, oh, it’s not a narcotic, it’s going to be OK," says a woman who gives her name as Nicole. "But little do you know that they all take a toll on your heart and on your breathing."

Nicole says some heroin users pill-shop, knowing which symptoms to mention so their doctor will prescribe something for anxiety or depression. Nicole says many patients addicted to heroin do have a legitimate need for these prescriptions.