As drug addiction soars in the United States, a booming business of rehab centers has sprung up to treat the problem. And when drug addicts and their families search for help, they often turn to Google.

But prosecutors and health advocates have warned that many online searches are leading addicts to click on ads for rehab centers that are unfit to help them or, in some cases, endangering their lives.

This week, Google acknowledged the problem — and started restricting ads that come up when someone searches for addiction treatment on its site. “We found a number of misleading experiences among rehabilitation treatment centers that led to our decision,” Google spokeswoman Elisa Greene said in a statement on Thursday.

Google has taken similar steps to restrict advertisements only a few times before. Last year it limited ads for payday lenders, and in the past it created a verification system for locksmiths to prevent fraud.

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In this case, the restrictions will limit a popular marketing tool in the $35 billion addiction treatment business, affecting thousands of small-time operators.

“This is a bold move by one of the world’s biggest companies, saying people’s lives are more important than profit,” said Greg Williams, co-founder of Facing Addiction, a nonprofit group that is an advocate for people struggling with addiction.