Authorities were called to the scene when customers at a sidewalk ice cream salon across the street from the bar saw the five victims fall to the ground.
"We got an emergency call at 5 p.m.," or 1500 GMT, said police spokeswoman Iris Seper.
She said officers later took the unidentified suspect into custody after witnesses said they had seen a man offering the victims drinks.
Investigators said it was unclear exactly what substance had been added to the drinks, but that it appeared to be some kind of narcotic. Seper said the victims drank freely, apparently unaware that anything was amiss.
The five - three men and two women, all in their 20s - were taken to several hospitals around Vienna, Seper said.
Police would not say whether the suspect was employed at the bar in Vienna's central Alsergrund district or had any other connection to the establishment.
It also was unclear whether the five victims knew each other and whether their attacker had singled them out or targeted them.
The bar where Sunday's attack took place is one of dozens of cafes, taverns and restaurants nestled in the Stadtbahnboegen - arched spaces beneath elevated tramway trestles.
The neighborhood is situated on the Guertel, a main thoroughfare ringing the Austrian capital.
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