Furosemide, sold under the name Lasix, can be purchased without a prescription and is used to treat high blood pressure and fluid buildup.
So why exactly is it banned?
https://sports.yahoo.com/innocent-guilty-robinson-cano-latest-mlb-player-deny-ped-use-201751127.html
Furosemide is considered a masking agent.
According to the JDA, Cano had an opportunity to explain the presence of a masking agent. The policy also reads: “The presence of a Diuretic or Masking Agent in a Player’s urine specimen shall be treated as a positive test result if the [Independent Program Administrator] determines that the Player intended to avoid detection of his use of another Prohibited Substance.”
It's a drug that masks PIDs. So it's on the banned list.
I'm not happy about this or anything, it's a "Wow... Him?" story. When an MLB player visits his trusted Dominican Republic doctor (or any doctor), you'd think priority one would be "don't give me a banned substance," unless that wasn't your priority.