These questions could (and should) equally well be asked of any new food, be it a GM food, a new conventional variety of potato or a new brand of processed snack. But, in fact, usually only in the case of the GM food, will any tests for allergic reactions, toxicity or general negative effects on health, be carried out before it is released onto the market.
For example, if manufacturers want to develop a new conventional (non-GM) potato, they could simply take a wild variety of potato and cross-breed it with any other variety of potato. Potatoes, particularly wild varieties, are extremely poisonous because they contain compounds called glycoalkaloids. The manufacturer must test the new variety of potato for glycoalkaloid content to make sure it is not poisonous, but that is the only test that is required before the new potato can go to market. In the past, some conventionally bred potato varieties have had to be withdrawn because the glycoalkaloid concentrations were too high. There is no requirement to test for any allergic potential (even though potatoes also contain patatin, to which allergies have been reported) or indeed any other health issues.
The question is really: should new GM crops be tested like other new food varieties, or should new food varieties be tested as thoroughly as GM foods?
Clare Mills (March 2005). Could genetically modified foods be a new source of allergens? SciDevNet (http://www.scidev.net/dossiers/index.cfm?fuseaction=policybrief&dossier=6&policy=56)