Steven Valdez thought he recognized the woman in the Medellín park. Chatting, the two realized they had matched on the dating platform Tinder. They exchanged numbers and made plans.

On their date last spring, he said the woman suggested that he try a typical Colombian dish — a creamy soup called ajiaco. She carried it from a restaurant counter to their table.

He had two spoonfuls, Mr. Valdez, 31, said. “And that’s the last thing I remember.”

Like scores of visitors to the Colombian city last year, Mr. Valdez, a travel blogger, said he was told at the hospital that he had ingested a powerful, potentially fatal cocktail of sedatives, including a drug called scopolamine.

Scopolamine makes its victims black out, and experts say it can also make them unusually open to suggestion — including agreeing to hand over a wallet or reveal passwords.