Laboratory tests on marijuana believed to have been laced with fentanyl found no traces of the powerful opioid, Brattleboro police said Thursday.

Last month, police responded to a report of an overdose and revived the patient with help of the opioid-overdose antidote naloxone.

The person told police that they had smoked marijuana and had not taken any opioids. Officers conducted a field test of the marijuana, which showed officials it contained fentanyl.

In a separate incident last week, police and federal agents seized marijuana suspected of containing fentanyl.

But in both cases, laboratory tests of the marijuana did not find fentanyl.

The original discoveries prompted police to warn members of the public that they should know the source and history of any marijuana they consume.

The department “stands by its previous public safety advisory that it is wise for consumers of marijuana to know the source and history of any marijuana they ingest,” police said in a Thursday news release.