Police in Hull have confirmed a case of spiking by injection and say they are also investigating a second incident.
Detective Chief Inspector Pete Thorp of Humberside Police said: “We understand that hearing of incidents of this nature in this area can be concerning for the public and we would urge everyone to continue to remain vigilant.”
Both plain-clothed and uniformed officers are patrolling busy nightlife hotspots in the area.
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Detective Thorp added: “We are also continuing to work with door staff and licensees, to build on existing working relationships and to help them spot the signs of anything potentially concerning, including spiking, early on.
“These signs include anyone acting or behaving suspiciously with another person who looks too intoxicated or has been separated from friends, or anyone trying to get someone out on their own, in a dark corner or secluded spot – or anyone leading someone into a taxi or unmarked car.”
It follows reports a woman in Preston was taken to hospital after she was injected with an unknown substance, triggering a police investigation.
The incidents come amid an increasing number of reports of drinks being spiked and separate claims that people have been drugged by injection.
Nottinghamshire Police said they had received 44 reports of spiking over recent weeks – with 12 of the cases involving victims saying they were spiked by a needle.