The Precautionary Principle basically says that we should not do something unless we are sure it will have no harmful or potentially harmful side-effects.
On the face of it, that may sound reasonable, but in reality it is a one-sided consideration, which completely ignores the benefits or potential benefits of any action/product/invention. The Principle does not weigh benefits against costs; it just says that if there are any costs at all, the action should not be carried out. For example, a new invention which could benefit millions of people, but also might possibly harm a few (people or other species) in the process, should be banned according to the Precautionary Principle.
If the Precautionary Principle had been enforced at the time of the invention of the wheel, the wheel would surely have been banned, especially if the people of that time had had the imagination to foresee all the death and destruction this invention has caused in terms of traffic accidents, contamination, environmental destruction, obesity, etc.
A more modern example is Genetically Modified Organisms (GMOs), which have the potential to substantially increase crop yields, to reduce the need for expensive and dangerous herbicides and pesticides, and to make food products healthier, tastier and more nutritious, as well. This should be good news if you are worried about hunger in the third world, protection of the environment, and/or your own health. Still, many people and organizations are opposed to the use of genetically enhanced crops, frequently citing the Precautionary Principle (1).
Every single invention (indeed every single action) between the invention of the wheel and the GMOs has some risk associated with it. If it were not for billions of brave persons willing to take a risk and willing to violate the Principle, we would all be living in the stone-age, walking around paralyzed.
Unless stone-age living is your ideal way of living, you should be very cautious in the application of the Precautionary Principle.
(*) Director, Institute for Advanced Development Studies, La Paz, Bolivia. The author happily receives comments at the following e-mail: email@example.com.
(1) There is not much else to cite, as, to date, there is not a single scientifically documented case of harm done by genetically modified crops.