Ellie Simpson says her sister felt “a pinch on the back of her arm” as she left a nightclub.
A man has been arrested as police investigate three reports of women being injected with needles in the last two weeks.
One 19-year-old woman said she was targeted as she left Stealth nightclub in Nottingham on 12 October.
Ellie Simpson said her sister, who does not want to be named, felt a “pinch on the back of her arm” before blacking out and being taken to hospital.
At least one other venue is also involved in the investigation.
Nottinghamshire Police said a local male had since been bailed but the force did not say which incident the arrest was in relation to.
Detectives are looking at toxicological reports and CCTV footage, it added.
Ms Simpson, 21, said her sister – who is a student from Derby – believes she was injected with a mystery liquid and had blood samples taken at the hospital.
She said: “I don’t think it’s quite yet sunk in what’s happened to her.
“It’s really frightening because I don’t know how you’re meant to prevent it.
“Obviously you can put your hand over your drink but how do you stop somebody stabbing you with a needle?”
Ms Simpson said her sister, who had recently moved to Nottingham to go to university, was still “in shock” and has not been out clubbing since.
“Normally she’s the type of person that would stick up for herself, so I think if it could happen to her it could happen to somebody who is more vulnerable,” she said.
Meanwhile, a group called Girls Night in Nottingham has encouraged women to boycott nightclubs on Wednesday 27 October.
They hope the boycott will “ensure the issue of spiking is taken seriously”, and are calling for more thorough searches of guests before they enter.
On Instagram, organisers wrote: “Girls should not have to feel unsafe and uncomfortable to go out and enjoy themselves.
“Together we can make a change.”
Stealth nightclub said it was “absolutely unacceptable” for women to live in fear of being spiked by needles on a night out.
A spokesperson said: “Here at Stealth, we have to date had two women who have reported feeling unwell and suspected they might have been spiked.
“Both were seen by our on-site medic, and we are currently liaising with police to aid in their investigations.”
The nightclub said reports of spiking were taken very seriously and staff would continue to carry out thorough searches and capture CCTV footage to aid police investigations.
Nottinghamshire Police is investigating two further incidents but details of these have not yet been released.
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