![dek](https://content.time.com/time/daily/2007/0711/mmm_atoz.jpg)
A single marijuana cigarette has the same effect on the lungs as smoking up to five cigarettes in succession.
The scientific bulletin of the year may be the stem-cell breakthrough. But 2007 provided a whole alphabet of big medical news. TIME's A-to-Z guide reviews them
By Coco Masters, Alice Park, Carolyn Sayre, Tiffany Sharples, Alexandra Silver and Kate Stinchfield
A single marijuana cigarette has the same effect on the lungs as smoking up to five cigarettes in succession.