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!"
Yup! And "injected intravenously" could actually be done one of two ways: with commas (incorrectly done here) or with en- or em-dashes. For example, "the substance that – injected intravenously – turns women into cats!" (en-dashes) or "the substance that—injected intravenously—turns women into cats!" (em-dashes) are both valid. I often tend to prefer en-dashes because I think they're easier to read than em-dashes and put more emphasis than commas on the idea that this is a bit of an aside. I think commas are just a bit too overloaded and that en-dashes add more flexibility to grammar.