Police in England said on Friday they had arrested three men, after several days of increasing alarm at reports that revellers had been injected with needles at nightclubs.
Young women and men, mostly students, have said they believe they were injected with drug-filled syringes at clubs and bars.
Women have for years been warned to be alert to their drinks being spiked with alcohol or sedative drugs while on nights out.
Longstanding fears about the safety of women in public places have also come under renewed focus after the high-profile murder of a London woman by a rogue police officer earlier this year.
Police have said there had been around 140 confirmed reports of drink spiking in September and October and 24 reports of injection.
Senior officer Jason Harwin of coordinating body the National Police Chiefs' Council said the reports were "very concerning" and the council was "working at pace" to "understand the scale of offending".
Police in Nottinghamshire county in central England said on Friday they had made two arrests in an ongoing investigation into "spiking" incidents.
The pair were suspected of conspiring to administer poison "with intent to injure, annoy or aggrieve".
The force said the arrests were not linked to any specific recent report of drink spiking or use of a needle.
Police in neighbouring Lincolnshire arrested a man on suspicion of possessing drugs with intent to administer them, though officers said no needle was involved.
Nottinghamshire Police said they had 15 reports of "alleged spiking by something sharp" since the start of this month, and 32 other spiking reports since early September.
Two other police forces said they were investigating claims of needles being used in attacks.
Students in dozens of university towns and cities have responded with plans to boycott nightlife venues next week, calling for tighter entrance security and better safeguarding.
More than 163,000 have signed a petition urging the government to make it compulsory for nightclubs to thoroughly search guests on entry.
Victims have reported passing out or feeling woozy shortly after experiencing sharp pain, and later finding apparent puncture marks on their skin.
Their accounts were discussed by the House of Commons home affairs committee on Wednesday, with its chair, Yvette Cooper, describing the alleged attacks as "vile crimes".
Some medics and drugs experts, though, have questioned what powerful sedative substance could be administered in such a way without the victim immediately noticing.
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