The internet is dramatically changing the way we live. But in few walks of life does it have the potential to change things more than in health. Online consultations, self-administered tests with the results sent to your GP by e-mail, remote video diagnosis and much more are the long-term future. But the short-term future is here already.

Already the internet has the ability to create "the intelligent patient", or someone able, if not yet to talk on equal terms with their doctor, at least able to talk on more equal terms. Information isn't necessarily knowledge. The knowledge and experience that comes from a doctor's training cannot be picked up overnight. But the net offers the chance to be vastly better informed - better able to understand the options and choices in healthcare.

Huge quantities of information about lifestyle and how to stay healthy are available. Even greater quantities are available about what your diagnosis means, what the prognosis may be, what treatments are available. All can now be accessed online from around the world. That patients are using it (as well as health staff) is not in doubt. Thirty per cent of all hits on the internet are now said to involve health-related issues.

Ten years, even five years, ago, the opportunities for the lay person to really understand medicine and health were limited. Looking back, childbirth was one of the few areas where patients were able to have a real impact on medical practice. The campaign for more natural childbirth, and for women to have more choice over the method of delivery, was one of the turning points in relations between doctors and patients. But women had nine months to prepare for childbirth. They were not,usually, ill. They had time to read and learn about the process.

For almost everyone else, formidable barriers stood between them and understanding. There were medical charities, self-help and pressure groups that provided leaflets and information for patients with a huge range of conditions from cancer to heart disease, and from Alzheimer's to Parkinson's. But the phone and post were the only means of accessing that. Those who wanted to dig deeper found medical knowledge buried in libraries and hidden in medical journals, where the jargon required expertise. There was no easy way to know what information was out there. Only the most dedicated had the time and ability to dig it out and digest it. The internet has changed all that.

HIV and Aids is arguably the first area where patients have used the net to learn about what has happened to them, to follow the research, to speak to others and to campaign about how their illness is perceived, researched and treated. Along the way they have changed the policies of pharmaceutical companies, drug licensing authorities, doctors and government.

Now patients with any longer-term disease or condition that you care to name can - at some cost in time and money, but at much less cost than in the days before the internet - get to know certainly as much as their family doctor about their condition, and, for the really dedicated, may even be able to speak on close to equal terms with their specialist.

That, of course, is the extreme. For most people, the ambition will be simpler: just wanting to know more. Either way, the internet is changing the doctor/patient relationship, and in ways that not all doctors, nurses and therapists as yet find easy. Many are coming to terms with it and realise that, used well, the internet can reduce patient demand. But there are still doctors whose reaction if you walk in with a sheaf of internet print-outs may still be a mixture of irritation and fear.

But if the internet provides a wealth of newly accessible information, the problem today is not too little of it but too much. Too much even for doctors to digest, let alone patients. There are, at the latest estimate, well over 100,000 health-related sites on the net. There are tens of thousands of less reputable health sites of one sort or another, and there are countless millions of pages of health information out there. Put the word cancer into a search engine such as AltaVista and you get 3m results. Narrow it to breast cancer and you still get 350,000.

In using the web to search for information on health it is crucial to remember two things - that even in this day and age, medicine remains as much an art as a science and that doctors, let alone patients, need help sorting the reliable from the unreliable.

Fifty years ago, the Index Medicus, the main index of medical research, ran to a couple of slim volumes. Today well over half a million new medical articles are published each year and the number of web pages on health is rising by at least 20m a month. No one on their own can keep up. Around the world, bodies such as Nice, the new National Institute for Clinical Excellence (www.nice.org.uk), are being created to provide guidelines for doctors on the best treatment for given conditions in the light of the latest evidence. There are bodies such as the Cochrane Library (www.update-software.com) that assess the quality of clinical trials. And the health community, worried about the quantity of downright misleading and sometimes outright fraudulent quackery out there, is attempting to develop kitemarks for reliable information - through organisations such as Health on the Net (www.hon.ch), Omni (www.omni.ac.uk) and Discern (www.discern.org). The range of kitemarks available is itself confusing, and it is likely to be years before any international standard is agreed.

Another problem for anyone seeking health information online is that the quality of sites can change rapidly. Good sites can become bad ones (though not, generally, the bigger government-funded ones). Bad ones can become good. And new ones are being added all the time.

Even by the time you read this, things will have changed. So what follows is much less a guide to sites for particular conditions than a guide to the sites which can point you to the places where it is likely you will find what you are looking for. As the net continues to change, these sites will remain useful starting points.

Email Nicholas Timmins on nick.timmins@ft.com