iOS 7 includes more animation than iOS 6. Apps fly open and closed, for example. Some people have said the excessive movement gives them motion sickness.
An Apple message board spotted by The Verge lists a number of symptoms, including "severe vertigo" and the feeling of "car sickness."
"I'm going on day 3 of total nausea," one complained.
"It's exactly how I used to get car sick if I tried to read in the car," says another.
"I had severe vertigo the minute I started using my ipad with ios 7," someone with a more extreme case wrote. "Lost the rest of the day to it... And not happy at all."
If you're sensitive to motion and animation, you can adjust your iPhone settings to tone them down. One user showed other motion-sick users how:
Some have gone through the hard work of downgrading their iOS.