South Africa's controversial Health Minister, Manto Tshabalala-Msimang is facing new calls for her resignation.
They follow newspaper allegations that she underwent a liver transplant while suffering from alcoholism.
The government says the reports are "false and speculative", and President Mbeki's office says he still has confidence in his health minister.
Dr Tshabalala-Msimang has - in the past - come under fire over her unorthodox approach to the HIV-Aids crisis.
Her emphasis on the use of garlic and beetroot for HIV sufferers brought her many critics.
But over the past fortnight, South Africa's Sunday Times newspaper has made startling allegations that the health minister was an alcoholic who jumped the queue to obtain a liver transplant earlier this year.
The paper has also said that as part of a five-month investigation, it discovered that Dr Tshabalala-Msimang was convicted of stealing from a patient when she worked as a medical superintendent at a hospital in Botswana 30 years ago.
The opposition Democratic Alliance is now calling for an investigation into claims that President Thabo Mbeki contacted the surgeons at a Johannesburg medical centre to insist that they approve a transplant for the minister.
The opposition leader, Helen Zille, says if this is true, it's "a disgraceful abuse" of the president's public position.
But the government is standing by the health minister.
It says the allegations are "false, speculative and bizarre", and there will be no investigation until evidence is produced.
The president's official spokesman says Mr Mbeki still has confidence in Dr Tshabalala-Msimang.
President Mbeki and Dr Tshabalala-Msimang have an association that stretches back more than four decades.
They were part of the same group of students which fled South Africa to go into exile in 1962.
Ten days ago, Mr Mbeki fired his deputy health minister, Nozizwe Madlala-Routledge, saying she had not been a team player and had made an unauthorised trip to an Aids conference in Spain.
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