Feburary 2020: Okay so, I somehow really overlooked this. It's actually for the rehab program, which as my 2019 update could include overdosed inmates.
Edit September 2019:
I believe this is purely to indicate a Overdosed state.
That icon is a part of the general mood bubble. It shows the prisoners current need (at least speculated to be part of it), in that example the prisoner needs drugs or a rehabilitation program. They will attempt to smuggle in drugs if this gets critical.
You can view the entire Prison's Needs if you have a Psychologist.
Mousing over a prisoner will show what need and how much they need it.
Needs marked Green are being satisfied currently, where as white ones are not.
You can see an entire overview of this in the reports in the lower right:
When the needs are low your prison may end up retaliating back and trying to escape or satisfy the need by smugging in drugs, or rioting.
When they're close to breaking point the icon will change to have spiked edges around it, showing it's critical. Typically when they break they try destroying stuff in retaliation.