Neither have I.
Never have I ever ran a marathon.

I might have? Though it was those events for kiddies so it wasn't very long.

Never have I ever flunked a test on purpose.

Neither have I.
Never have I ever eaten cat food.

I have! The dry kind, so it was crunchy. It tasted sort of fishy and didn't have a lot of flavor to it.

Never have I ever played spin the bottle before.

Neither have I.

Never have I ever failed a drug test.

Neither have I, considering I've never had a drug test before.

Never have I ever smoked.

I have.
Never have I ever been in a hot air balloon.

Neither have I, I think?? Don't remember.

Never have I ever started a food fight.

Neither have I.
Never have I ever slept in a teepee.

Meeeee neither.
Never have i ever eaten magic mushrooms.....contrary to appearances

Well I've eaten them while playing as Mario on a video game console, does that count?

Never have I ever mixed cement.

I have. For furnace bases and side walks
Never have i ever mugged anyone

Neither have I.

Never have I ever spat a mouthful of drink into someone else's face, whether by accident or on purpose.

Me neither.but I have thrown drinks at scumbags.
Never have i ever been in prison

Neither have I.

Never have I ever eaten at a fancy French restaurant.

Neither have I.

Never have I ever gone scuba diving.

Neither have I.

Never have I ever accidentally or purposefully overdosed.

Neither have I.
Never have I ever thrown up on someone.

Neither have I, I think.

Never have I ever stayed up for more than 24 hours.

I have.
Never have I ever swung from a chandelier.

Neither have I.

Never have I ever visited Australia.

I have. Had a pretty good time there.

Never have I ever done a back flip.

Neither have I... successfully, anyway!
Never have I ever gotten a tattoo.

Neither have I, though I kind of want to. Just couldn't decide on a design that I like enough to get inked on my body.

Never have I ever walked a dog.

I have and have some interesting stories to go along with those walks. I miss having a dog.

Never have I ever been to Ireland.