TB infections can develop in HIV-positive patients
South Africa's health department has launched a probe after some batches of two tuberculosis drugs were removed from hospital and clinics to be tested.
Health officials want to establish the shelf-life of the drugs used to treat thousands of patients in the country.
The manufacturer, Pharmascript, has replaced stock older than four months old and continues to supply new drugs.
The investigation comes within days of some anti-retroviral (ARV) drugs being recalled after a packaging mix up.
HIV and Aids patients have been urged to urgently seek medical advice.
Initial tests showed that the TB drugs lost their potency over a period of time.
"If we're getting drugs that are not of quality... we're not going to get the results that we require," Health Minister Manto Tshabalala-Msimang said.
"Further tests are being conducted on the stability of these products," she said.
Pharmascript said it supported the testing process and was co-operating fully.
People may carry the TB infection for many years but only develop the disease when they develop HIV, which suppresses the immune system.
Certain types of Nevirapine, used by HIV-positive pregnant women, and Adco-Zidovudine (AZT) were recalled by Adcock Ingram over the weekend.
According to South African HIV campaign group Treatment Action Campaign, Adcock Ingram supplies ARVs to the private sector and has recently been awarded a tender to supply medications to state hospitals.
"We are concerned that the recall will undermine public faith in the quality and safety of medicines and are worried about the potential impact that the recall will have on the availability of life-saving ARV medications."
South Africa has one of the world's highest incidences of HIV.
One in five adults are reported to be infected and an estimated 500,000 people are infected each year.
Correction 15 August 2008: An earlier version of this article stated that the two TB drugs referred to were sub-standard. This has been changed to make it clear that they were potentially beyond their shelf-life.