Off-label use is standard in the treatment of cancer and AIDS.
The FDA approves medications in specific doses for specific indications. Prescription for any other purpose is considered "off label" (see here, here, and here). Physicians have the authority to prescribe any medication for any condition (assuming, in the process, liability for their action) for any reason. Boodman's article discusses the degree to which this practice lacks any scientific support, since many such off label scrips are written without the advantage of clinical research to justify them.
This is an area that carries both risks and benefits, especially for people living with HIV. It's not enough any more to accept a scrip and (if you are lucky) go to the pharmacy to get it filled. These days you need to know what the medication is approved for as well as what your physician is prescribing it for. "Why?" is the new black in physician-patient relations, as in "why are you prescribing this medication for this indication?"
I know this sounds preachy/teachy, but I don't quite know any other way to say it. As Boodman points out, off label is not the same as contraindicated. On the other hand, there may be risks and/or unknowns that you need to be aware of.