Although the problem seems to be confined to just one pharmacy in California, the fact that the bottles carried counterfeit labels - and yet contained genuine drug that could potentially cause harm to patients - underscores the serious of the situation.
In a letter to pharmacists, GSK noted that two 60-count bottles of Combivir tablets actually contained Ziagen (abacavir sulfate) tablets.
The company said it was concerned because around 8 per cent of patients who receive abacavir develop a hypersensitivity reaction which can in some cases be life-threatening.
Patients who are prescribed Ziagen should receive advice from their doctors to make them aware of the risk and, crucially, ensure that they stop taking the drug immediately.
But patients prescribed Combivir would not get that warning.
GSK's letter notes that there have been no adverse reactions in patients resulting from the tampering, and that it is "working with the FDA to investigate".
The fact that the Ziagen in the bottles appeared to be genuine, even though the labels were fake, is particularly bizarre and for the moment neither GSK nor the Food and Drug Administration (FDA) have been able to shed light on who is responsible for the tampering, or indeed why it was carried out.
The counterfeit labels identified the bottles as Lot No. 6ZP9760 with expiration dates of April 2010 and April 2009, said GSK, adding that the two products are easy to tell apart: Combivir is a white capsule-shaped tablet engraved with GX FC3, while Ziagen is a yellow capsule-shaped tablet engraved with GX 623.