Red light therapy can help with acne, hair thinning and mild skin aging, but companies selling at-home devices often promise more than the science demonstrates, according to a review published in September.

Masks, wands and full-body panels that were once mostly found in dermatologists’ offices are now sold for homes and gyms, with promises of smoother skin, thicker hair and faster recovery.

The researchers compared what studies actually show with what top device brands claim. They found that the light does help with acne, hair loss that runs in families and mild aging from sun damage.

But the results were small, took weeks or months to show up and only lasted if people consistently used the device. The review also pointed out that marketing promises like “overnight” clear skin or permanent hair regrowth go beyond what any study has shown.

The treatment works by shining low levels of red light onto the skin. Scientists believe the light widens blood vessels, which brings more blood and nutrients to the skin and hair roots.

Some studies also show it helps the body produce more collagen, the protein that keeps skin firm.

Doctors already use red light in clinics with a special cream to treat early skin cancers, but red light on its own does not destroy skin cancer.

Hair growth has the most proof behind it. Using a device regularly for a few months can help thinning hair grow back, but the hair stops growing once people stop, Dr. Zakia Rahman, a clinical professor of dermatology at Stanford Medicine, said.

“There’s real evidence that shows red light can change biology,” Rahman said in a press release.

However, she added that it is not a fix for everything.

She also said it likely cannot regrow hair where the roots are already dead, although no studies have settled that question.

The evidence for other uses is weaker. Studies on healing wounds have mixed results and there is little proof that red light helps with sleep, workouts, chronic pain or dementia.

“The data is somewhat conflicting and there are a lot of caveats,” Dr. Nour Kibbi, a clinical assistant professor of dermatology at Stanford, said.

Where people get the treatment also matters. Machines at clinics are usually stronger than the ones sold for home use and most buyers don’t know how strong their device is or how long to use it.

While some devices have been cleared by the Food and Drug Administration, that mostly guarantees safety, not results.

The main safety rule is to never shine the light directly into the eyes.

For people thinking about buying a mask or panel, it comes down to what they want it for. Someone using it consistently for thinning hair or mild acne may see small but real changes.

Someone expecting it to fix everything may end up wasting their money.

Scientists still do not know the right dose, and larger studies are needed.