I know several people who have done well on wheatgrass fasts.
The key is to use either a powder or fresh grass that has been grown in highly mineralized soil and soil that has good natural drainage. The tray kind is notorious for growing mold. Most people that think they are experiencing die off symptoms form the wheatgrass are actually experiencing mold reactions due to tray grown grass.
Barley Gold is a superior barley powder. Lots of excellent testimonials.