"Uhh. Well it's like...a lubricant." Nice mismemory, Lars.

He reaches forward, grabbing Lauren's dick softly, giving it a single pull. "So prolly like this?" He does that a couple more times, coating it. He thinks it's been enough time since his orgasm that his dick shouldn't be annoyingly sensitive.

Of course, if Lars was actually mature enough to not be embarrassed out of his mind when the shopkeep was explaining how the sex aid worked, Lars would have remembered this correctly. He remembers that you can eat it, but it's a weird slime, right? So why would you want to? Lars's logic is that you would only do something as gross as eat weird drug lube if it was more potent that way.

So, the obvious answer in his mind is topical application. Which, incidentally, is actually what makes the aphrodisiac more potent.