Mayor Martin Walsh says that a spike in suspected overdoses from heroin and other opioids is cause for alarm in hard-hit neighborhoods, especially Dorchester, where a pre-Christmas rash of incidents prompted health officials to issue a public warning.

The Boston Public Health Commission announced on Dec. 23 that there had been 32 suspected overdoses in Boston over a two-day period the previous weekend (Fri., Dec. 19-Sat., Dec. 20). The biggest concentrations were in downtown Boston, East Boston, and Dorchester.

Since that time, there have been additional incidents, including two people who were found dead on Saturday evening in the hallway of a South End public housing building where drug activity has been an acute problem. The unidentified men apparently died of overdoses, according to preliminary reports from Boston Police.

Last Friday, Walsh told the Reporter that the spike has brought new urgency to a longtime problem.

“I think in some cases we have heroin that’s being laced with some type of other drug, so a lot of people are overdosing because of that,” said the mayor. “It was reported in one paper that we have seen an increase in heroin use, but it’s always been there, we’re just seeing it more. It’s a very difficult issue to tackle. We still need to do more about prevention and cutting back on the number of heroin addictions — and on the pills. The pills are what leads to the heroin.”

Walsh said that the 32 Boston EMS calls for overdoses last weekend — while alarming — are probably just a starting point. “Those are just the ones that are reported,” he said. “People die of overdoses every day and a lot of times they’re not documented.”

In an advisory issued on Dec. 23, public health officials said that the Boston Police Drug Unit was investigating the cause of the spike in overdoses by conducting so-called “controlled buys”— in which police test the potency of drugs being sold on the street.

The Health Commission said last week that 761 doses of the opiate antidote Narcan have been administered to patients by Boston Fire, EMTs and trained family members in the city so far this year. Over the same period in 2013, 499 doses of the drug were used. According to BPHC officials, 19 doses of Narcan were administered to individuals in Dorchester alone between Dec. 19 and Dec. 28.

“Unfortunately, our city has not been spared the increase in heroin-related illness seen throughout our region,” said Dr. Huy Nguyen, interim executive director of the Boston Public Health Commission. “This recent spike in suspected overdose cases highlights just how important it is that substance users, their families, and the community at large know how to identify overdose symptoms and prevent overdose deaths.”

Nguyen said the agency was stepping up instructions on the proper use of Narcan at city homeless shelters and through other outreach programs.