Can the damage from smoking be reversed?

Some of the effects of smoking on the digestive system appear to be of short duration. For example, the effect of smoking on bicarbonate production by the pancreas does not appear to last. Within a half-hour after smoking, the production of bicarbonate returns to normal. The effects of smoking on how the liver handles drugs also disappear when a person stops smoking. However, people who no longer smoke still remain at risk for Crohn's disease. Clearly, this question needs more study.

source: http://digestive.niddk.nih.gov/ddiseases/pubs/smoking/index.htm