There are actually two commas in there, and adding just one is still one too short. The original is:
"This, my friends is the substance that, injected intravenously turns women into cats!"
The sentence should be:
"This, my friends, is the substance that, injected intravenously, turns women into cats!"
See, this is why I hedged my bets. That wasn't the comma that threw me off. I didn't even see that one on first read-through. I was focused on the "the substance that, injected intravenously, turns women into cats!"