Doctors have called for more research into a type of laser treatment used on people with acne.
A growing number of people are having laser therapy for acne
Researchers in the United States tested pulsed dye laser or N-Lite therapy on 40 people with facial acne.
Writing in the Journal of the American Medical Association, they said the treatment had little impact.
Manufacturer ICN Photonics, which is based in Llanelli, expressed surprise with the finding and said it contradicts other studies.
The technique involves using a laser to deliver a yellow light to destroy the bacteria that causes acne.
The yellow light interacts with a substance inside the bacteria called porphorine to create oxygen which destroys the bugs. It also promotes the growth of collagen to prevent scarring.
The therapy was originally marketed in the United States as an anti-aging treatment.
But a few years ago doctors discovered that it may be an effective treatment for acne.
Last year, doctors at Hammersmith Hospital carried out tests on 41 patients.
They found that the therapy reduced acne in many of the patients. Ten were completely cured. The findings were published in The Lancet.
Doctors from the University of Michigan Medical School, who carried out this latest study, said their findings highlighted the need for more research into the therapy.
"The fact that our study does not substantiate the positive results recently reported [in other studies] is not an indictment of laser therapy for acne in general and does not necessarily rule out the possible role of this particular pulsed dye laser.
"However, it does suggest that additional well-designed studies are needed before the use of the pulsed dye laser becomes a part of acne therapy."
The UK Acne Support Group said the jury was still out on pulsed dye laser therapy.
"We have been hearing mixed results," said Alison Dudley, its chief executive.
"Some have been very positive while others have been left disappointed.
"We would advise patients and their families to explore the options fully before having this treatment."
Peter McGuiness, chief executive of ICN Photonics, said the company was in the process of carrying out further research which would back up last year's results from Hammersmith Hospital.
But speaking to BBC News Online, he said he was surprised by these latest findings.
"The results from clinics in the UK and around the world have been excellent. Our experience has been that the therapy is comparable to if not superior to antibiotics. I am surprised by these findings."