This was brought up a few months ago in the QAA pod (episode 166, starting at 10:39), in response to the police-based rumors (which then were confirmed to be bullshit) of someone getting stuck by a random needle. Annie Kelly came on to talk about the needle attack cases that were happening in the UK, and added a lot of skepticism to this overall.

I (along with Annie) genuinely believe that these victims have been spiked, but I find it very unlikely that it was actually through a syringe. Some points:

• Spiking is most often done by adding more alcohol to a drink - this is easy for the spiker and gives them plausible deniability, and cops hand wave these cases away by saying the victim just couldn't handle their liquor.
• A syringe, by contrast, needs to be snuck into a club and precisely stabbed into the victim's vein, found in the dark and motion-heavy club environment, no less
• Getting stabbed by a needle provokes an innate physical reaction to move away from the source, limiting the amount of drug that could be potentially injected
• Over the hundreds of reported cases, only a single crime has been reported (a robbery), while traditionally spikers choose a target to then commit some crime (usually sexual assault). Drugs aren't free and it seems unlikely to me that people would use them just to give other people like worse hangovers
• There were similar rumors of people going around clubs during the AIDS epidemic sticking people with HIV-positive needles, which turned out to be bullshit

Don't get me wrong, I think it's definitely possible that someone motivated could pull off a needle spike attack, but an epidemic of unconnected attacks across Europe without follow-up crimes or evidence of drugs in the system seems incredibly unlikely.