Chris Kenny, the county's director of public health, warned that a batch of the drug being circulated was three times stronger than normal.
He said the six who died were part of a drug rehabilitation programme.
Potential users as well as hospitals, pharmacies and GPs have all been alerted about the risk.
Mr Kenny said all of those who died within the past week had been on the "road to recovery" as part of the Crime Reduction Initiatives (CRI) programme.
He said: "We're aware there's some heroin circulating in the community that's two or three times the purity.
"What's happened is [CRI] service users have obviously been taking this and have inadvertently taken a level that is dangerous to them and in six cases they have died.
"We're treating it as a public health emergency."
Mr Kenny confirmed that Nottinghamshire Police was investigating the deaths and the CRI has begun an internal inquiry.
Drug users, their friends and families, can call a 24-hour hotline on 0115 896 0798 for advice about the drug.
A "public health emergency" has been declared in Nottinghamshire after six people died in the county within a week after taking unusually pure heroin.