I had this same question and there are a lot of discussions about this on forums. Some say that because the iPad charger is twice more powerful than the iPhone charger, using the iPad charger will overload your battery and cause the battery to die faster than normal. Others say it should work fine.
It was very confusing so I went on to find out the real answer.
First: on the iPad charger description on the Apple store, it lists the iPhone as a supported device.
Second: when I measured the wattage, here was what it came out:
- When I charged my iPhone using the iPad charger, the consumed wattage was around: 6 watts.
- When the iPhone was nearly full, the consumed wattage dropped to below 1 watt.
- When I charged my iPad using the iPad charger, the consumed wattage was around: 12 watts.
=> It looks like the iPad charger knows the iPhone and iPad and draws the correct current to charge the iPhone and/or the iPad.
Third: I called Apple Support and was told, it is fine to use an iPad charger to charge the iPhone.