"I'm sure they're glad you at least tried." Unless they're jerks and deliberately made the drink gross and not for standard humans. He flags down one of the bartenders and asks for a coffee.

One weird look and a couple of minutes later, the tall being behind the counter serves up a very large mug of what looks like coffee. And in order to prevent a potential second incident, Venom's sniffing and sipping the beverage on Ianto's behalf. It smells and tastes like coffee despite no visible brewing process.

"Here. I believe this one is safe to drink." No ill effects, just plain old coffee. Hopefully.