Had an encounter with a forgotten beast a while ago. Its extract caused dwarves to pass out, but no permanent damage - those that happened to walk over it usually were carried to the hospital, spent a few days there, then got up and walked out unharmed.
However, one of my military dwarves took a shot of extract full-on. His health page said that he was unable to breathe, so I was fully expecting him to suffocate soon and didn't spend too much time worrying about him. But he's still taking up a bed in my hospital, unconscious and unable to rest. Looking at his health history page, he was first brought to bed nine months ago.
I had attempted to get him to rest by deconstructing his bed and getting him moved to a new one, but as soon as he was placed in bed the second time I get the message Urist McWounded cancels rest: Interrupted by Forgotten Beast. I look at his inventory and see several pages of forgotten beast extract covering him. Of course, because he's not resting no dwarf will clean him, though they seem to be feeding and watering him regularly.
According to his health status, he's paralyzed, winded, cannot breathe, and just about every part of his body is rotten or swollen with blood, but he's ecstatic.
I'm imagining this bloated, wheezing dwarf laying in bed, smiling at the ceiling with glassy eyes, unable to move but high on FB extract.
Since he hasn't suffocated to death in nine months I'm working on irrigating my hospital in the hopes of getting him clean and back on his feet at some point.