Lab testing has found natural fly sprays are nowhere near as effective as those with synthetic ingredients.
Consumer NZ has compared 10 different products and also discovered the sprays that strike the fastest aren't necessarily the best.
The only thing more annoying than the sound flies make is trying to get rid of them.
"We'd had some complaints from members that fly sprays didn't seem to be working that well anymore, so we decided we'd put them to the test," said Consumer NZ CEO Sue Chetwin.
Researchers at Consumer NZ tested 10 products in specially built chambers containing 20 houseflies each.
Six out of 10 sprays killed all the flies within 24 hours.
Mortein Fast Knockdown and Black Flag Rapid Kill both worked the fastest, felling 90 percent of the flies in less than a minute, but they released double the spray, so you'll go through a can faster.
The cheapest product was Zap, which achieved a 100 percent kill rate, but took five times longer.
Raid Earth Options fly spray, which contains natural ingredients, took 26 minutes to knock down half the flies, but 24 hours later, 80 percent were still alive.
"The natural ones, in fact, were very ineffective," said Ms Chetwin. "So if you want to go down the natural track, then you probably won't get a very effective spray."
Where you aim also makes a difference.
Ms Chetwin says: "You have to be focused, so actually do spray it onto the flies, rather than just generally spraying it around a room.
"And that's why using automatic dispensers may also be ineffective… the spray can dissipate without making contact with the flies... and even if it does... it may not be enough to knock them down or kill them."
And if you simply can't decide on which product is best, one works all the time and straight away - the trusty old fly swat.