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Stem-cell research is one of science’s most promising fields. Scientists believe stem cells may someday be used to repair or replace tissues and organs lost to age or disease, though they say effective therapies are still years away.

Hundreds of clinics, however, have already popped up across the country offering expensive treatments that are unproven by clinical trials and largely unregulated by the government. Critics say these treatments have caused patients physical and financial harm.

Here’s what consumers need to know:

What are stem cells?

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Stem cells are a class of unspecialized cells with the ability to develop into different types of cells, such as brain, blood and muscle cells. In many tissues, they act as “a sort of internal repair system, dividing essentially without limit to replenish other cells,” according to the National Institutes of Health.

But all stem cells aren’t created equal – they have different capabilities. Embryonic stem cells, which are derived from three- to five-day-old embryos, are “pluripotent.” That means they can become any of the more than 200 cell types in the body.

In comparison, adult stem cells come from already existing tissues and organs, and their adaptation is much more restricted. Researchers have figured out how to alter some adult stem cells in the lab to make them behave like embryonic stem cells, making them an important research tool.

Many of the commercial stem-cell clinics selling unproven treatments say they use adult stem cells called mesenchymal cells, which are found in marrow, fat and blood. While those can turn into several cell types, experts say there’s no evidence that they can travel through the body, finding and fixing an array of problems, as some clinics claim.

What are stem cells’ proven uses?

Transplants using stem cells from bone marrow or blood have been used for years to treat diseases such as leukemia, lymphoma or multiple myeloma. Such cells replace blood-producing stem cells in bone marrow that have been destroyed by disease or harsh cancer treatments.

Some of these treatments, especially in children, use placental or umbilical cord blood. These blood products are the only stem cell therapies approved so far by the FDA.

What diseases might be treated in the future using stem-cell therapies?

Researchers are studying stem cell treatments for an array of problems, including heart failure, brain cancer and neurodegenerative diseases such as ALS, or amyotrophic lateral sclerosis.

Age-related macular degeneration is among the most promising targets, partly because “the eye is an accessible part of the central nervous system and you can look in and see what you are doing,” said Sally Temple, co-founder of the Neural Stem Cell Institute in Rensselaer, New York. Yet safe and effective treatments for eye disorders and other diseases are years down the road, she added.

Many stem-cell clinics market treatments for arthritic knees as an alternative to surgery. However, a study led by Cleveland Clinic orthopedic surgeons found that “current evidence does not justify the rapid rate of growth for these therapies,” said co-author George Muschler.

What risks do stem-cell clinics pose to patients?

Generalizing about safety is difficult because how clinics process and administer stem cells varies, as does the expertise of the people performing the procedures. Another factor is where the stem cells are injected or infused. Many scientists are leery of any treatment – such as stem cell treatments – for which safety and efficacy have not been rigorously tested in human trials. But some stem cell procedures pose a greater chance for harm than others.

FDA officials say they are focusing their enforcement action on clinics offering “high-risk” procedures, such as injections into the brain, eye and nervous system.

At least four cases have been reported in medical journals of patients going blind or suffering severe vision impairment from stem-cell injections. In addition, two deaths have been documented in Florida.

For many patients, the harm is not physical, but financial. One former FDA commissioner said “the main risk is separating people from their pocketbooks.”

Who is supposed to regulate the stem-cell treatment industry?

The FDA issued guidelines last year to clarify which types of stem-cell treatments are in essence unapproved drugs and require agency approval. They include treatments that more than “minimally manipulate” patients’ cells and those that use stem cells in a way different from their original purpose – for example, a fat stem cell to treat a neurological disease. Many of the fat-derived stem cell treatments require FDA approval under the policy.

Critics say state medical boards, state legislatures and state attorneys general should also be helping rein in the runaway growth of unproven stem cell treatments.

What should consumers watch out for?

David Parke, chief executive of the American Academy of Ophthalmology, warns consumers to be very careful “if someone says they are involved in research trial and asks you for money.” The FDA urges patients to ask stem-cell practitioners whether their treatments have been approved by the agency or are being studied under an investigational new-drug application, which is a plan submitted to the agency and reviewed for safety.