How do I get the 'Medical Expert 2' Tonic?

Did you kill Sander Cohen the first time he showed his face in Fort Frolic?

If so, that was a bad idea.

First off, for completing his 'masterpiece', he would have rewarded you by unlocking the Tonic, but not if you kill him before he gets down there and unlocks it. Similarly, killing him now means you won't be able to get into his place in Olympus Heights later in the game - where you'll find, amongst other nice loot, a 'Power to the People' weapon upgrade machine.

If you wait long enough he will look at his masterpiece and unveil the tonic. You can then collect it and kill him and get the key to his music box.