This is a real possibility in two ways;
(1) the opportunity to improve the nutritional value of food and to increase antioxidants in food; and
(2) the use of specially constructed GM plants as "factories" for producing medical substances; they are cheap and easy to grow on a large scale, and so can reduce the costs of drugs.
For example, if a vaccine against HIV were to be developed, there are currently no facilities for producing it on a large enough scale for the Western World, let alone Developing Countries where the need is much greater. Possibly thousands of kg would be needed each year and we simply do not have that capacity at present.
It is important to ensure that GM plants used for making medicines are segregated from food crops. There are many ways to do this: geographical (growing GM medicine plants in different areas from food crops), containment (growing them only in greenhouses), or using non-food varieties (e.g. white tomatoes, blue potatoes) so that it is easy to distinguish the different kinds of plants.
Familiar foods like tomatoes containing antioxidants and other substances that may have anti-cancer properties or help prevent cardiovascular disease can be modified to provide greater and more effective quantities of the beneficial compounds without increasing the amount to be eaten.
Yet another possibility is to remove the allergens from peanuts and other foods that can provoke severe, even lethal, effects in susceptible patients. It might be possible to modify the offending gluten protein in wheat for sufferers from coeliac disease.
H.I. Miller (21.5.2004) Reaping What Biopharming Sows (http://www.techcentralstation.com/052104D.html)