A 55-year-old man recently presented to a Brisbane hospital with a black hole in his temple after applying an unproven alternative medicine known as "black salve" to a lesion on his face, which he believed was cancerous.

Writing in the Med-ical Journal of Australia, doctors from the Princess Alexandra Hospital said the man had been applying the unlicensed product, which is sold online as an alternative skin cancer treatment, to his face for four months.

The doctors, Natalie Ong, Eric Sham and Brandon Adams, said black salve preparations often contained an alkaloid derived from bloodroot and zinc chloride which could both eat away at tissue, leading to significant scarring and disfigurement.

"In the absence of a biopsy, some patients may commence alternative treatment before attaining a diagnosis of skin cancer, and a very real risk of recurrence and metastasis (cancer spreading) remains. As a consequence, there may be delays in diagnosis, and it may be difficult to identify the primary site of malignancy," the doctors wrote.

"It is imperative for health professionals to recognise that these unlicensed products may lead to adverse outcomes, and for consumers to realise that alternative therapies that have been described as natural are not necessarily safe or, by any standard, risk free."

President of the Cosmetic Physicians Society of Australasia, Gabrielle Caswell, said it was disturbing that black salve was being promoted on the internet in a scam that could cost lives.

“Skin cancers are a widespread and increasing health problem, particularly in Australia. The use of black salve has the potential to cause many problems including delayed diagnosis and treatment of cancers, with potentially fatal results,” Dr Caswell.

In 2012, the Therapeutic Goods Administration warned people to avoid black salve, which is also known as red salve, Cansema, or Bloodroot.

"The recent use of products marketed as containing 'Black salve' in Australia has resulted in serious harm to the skin of three Australian consumers who used the black salves for various skin conditions including the treatment of a skin cancer," the TGA statement said.

"The TGA is not aware of any credible, scientific evidence which shows that any black or red salve preparation is effective in treating cancer."