Stem-cell research has always been long on promise, but in recent weeks it has seemed to forge ahead. Last month British scientists announced the first successful transplant of an organ grown from the master cells - a section of windpipe known as a bronchus. And on Tuesday a German team reported that a stroke patient had received stem cells in a clinical trial for the first time.

While these advances have used adult stem cells, research using those taken from embryos is also progressing fast. In August a US company produced blood from embryonic stem cells, and another is poised to begin trials of a spinal cord injury treatment.

For all their potential, however, few stem-cell therapies are yet ready to be given to patients. The bronchus transplant and the stroke treatment are being offered only on an experimental basis, and embryonic approaches have not even reached that far.

There are real risks involved in injecting most types of stem cell into the body: they can seed cancer, and infection and immune rejection are further hazards. Given the expense of these therapies, it is also imperative that their effectiveness is proved before patients or health systems are asked to pay for them. Their current place in medicine is in properly supervised clinical trials. Yet, as the International Society for Stem-Cell Research (ISSCR) noted this week, novel therapies that have not cleared this hurdle are already being sold directly to patients. Many are aimed at those with incurable and debilitating conditions, such as spinal paralysis or multiple sclerosis.

Analysis by the University of Alberta of therapies marketed on the internet, found that most lack scientific credibility. The vast majority over-promise results and offer no substantiation for their claims in medical literature, while glossing over risks.

It is understandable that some gravely ill patients want these treatments regardless. They do not have time to wait for lengthy trials. Their desperation, however, makes them vulnerable to exploitation and heightens the case for regulation. In the UK, and in most of Europe and North America, good controls exist: it is illegal to conduct unlicensed therapy or to start unapproved trials. Countries such as India, however, allow operators to sell stem-cell treatments, or to run so-called trials that are neither well-monitored nor safe.

The ISSCR is thus to be commended for publishing a handbook for patients (available at www.isscr.org) which gives them a few tools with which to assess the claims of stem-cell therapies. It also wants countries that lack regulatory systems to develop them and is offering its help. These are sensible and practical measures, which should help patients and governments to identify and avoid shady practitioners.

A wider note of caution, though, is still needed when it comes to the promise of stem-cell research. It must be understood that tissue transplants using all types of stem cells are generally technologies of the future. There are some exceptions - the bronchus case is a genuine advance - but they do not mean that new kidneys, pancreases or spinal cords are around the corner.

For the time being, the immediate potential of stem cells, especially embryonic, is in a different direction. This week, two American groups grew motor neurons from embryonic cells, which they are using to study the development of motor-neuron disease and to screen new drugs. It is as just such a research tool that stem cells will be useful first. Effective and safe transplant technology will generally take a lot longer.

These caveats do not mean that it is wrong for scientists to get excited about stem cells, or for governments to back them with money. They have real potential. Patients, though, need to understand that this research will take time and beware of claims that stem cells are already transforming medicine.

Mark Henderson is the Science Editor of The Times

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