Didn't understand the presciption pills (spoiler)

The killer gives Harry the prescription pills, we see Harry with them, but never see him take any. At one point, the bad guy borrows the 'mold' guy's white uniform, and seems like about that time we see him handling the bottle of pills.

Later Harry's ex sees he has the bottle of pills, and she gets really upset. A scene near the end (around the table?) has the killer saying something to Harry about the pills, and how maybe they worked?

I haven't a clue what the medicine was. What was it? Why was the bottle shown being handled by someone else? What was the killer's intent when it came to the medicine? Did you get the impression Harry was or wasn't, taking the pills?

Thanks