First, the vast majority (probably 99.9 percent) of people who get a headache after drinking wine (which is more common after red wine) are not suffering because of the sulfites; it's from other substances in wine, especially the alcohol. These people should drink less, and with food. Also, taking 400mg of ibuprofen about a half-hour before you drink will often prevent a headache.

A small number of people are truly allergic; these generally are people with asthma or those on steroid medication who develop rashes, abdominal pain or sever breathing problems when exposed to sulfites. These people must be very careful--sulfites are present in most dried fruits, jams, baked goods, canned vegetables, shrimp, pickled foods and many other foods in addition to wine (where it is higher in white wines than reds).

In wine, some sulfites are present naturally and some are added to prevent bacterial growth. Wines from around the world contain sulfites, but only when sold in the United States do they have to carry a label saying 'contains sulfites'. Most wines contain less than 100 parts per million (ppm) of total sulfites; in comparison, dried fruits may contain 500 to 1000 ppm.

For the small percentage of people really allergic to sulfites, there are a few wines made without added sulfites; a wine with no added sulfites can be called a 'no sulfite wine' if it contains no more than 1 ppm. These wines often have unusual aromas, are very perishable, should be stored on their sides in a refrigerator and should be drunk within 12 to 18 months of release.

(From Wine Enthusiast magazine)