So, theories abound about what was in the syringe. Initially it seems that it's some sort of suicide cocktail. This theory has effectively been shot down by Joss.
Tim Minear has alluded that it's some sort of drug that will kill anyone (reavers) who have sex with her. This theory is so bizarre and gross that I'm personally hoping it was some sort of dark joke.
Other theories involve the contrasting ideas that she's terminally ill or that she has discovered the fountain of youth and it's some sort of anti-aging miracle.
What's your take?
Feel free to reference other episodes when discussing the character ~ I'm going to assume we don't require spoiler alerts in a re-watch. ;)