Maybe it doesn't work on donkeys.

2 seconds sound clip from the Shrek 2 movie soundboard.

You can hear this line at 00:49:42.038 in the Blu-ray version of the movie.

Quote context

[...]

- Aha! See, you're allergic to that stuff. You're gonna have a reaction. And if you think that I'm gonna be smearing VapoRub all over your chest, think again!

- Boss, just in case there is something wrong with the potion, allow me to take the first sip. It would be an honor to lay my life on the line for you.

- Oh, no, no, no. I don't think so. If there's gonna be any animal testing, I'm gonna do it. That's the best friend's job. Now give me that bottle.

(Donkey drinking potion)

- How do you feel?

- I don't feel any different. I look any different?

- You still look like an ass to me.

- Maybe it doesn't work on donkeys.

- Well... Here's to us, Fiona.

- Shrek! You drink that, there's no going back.

- I know.

- But, no more wallowing in the mud?

- I know.

- And no more itchy butt crack?

- I know!

- But you love being an ogre!

- I know!

- But I love Fiona more.

[...]