Does "injection drug use" always imply the use of illegal drugs?

From a news report:

Among those with both HIV and schizophrenia, the population was mostly male (75%), living in an urban setting (91%), had a history of injection drug use (75%), and was at least once on anti-psychotic medication (49%; P <0.001). The population of patients with schizophrenia, but not HIV, reported lesser rates for injection drug use (20%) and antipsychotic medication (39%).

I'm unsure whether they mean "the use of illicit (narcotic) drugs" or just "the use of any active substances, including therapeutic, via the injection route". Is there a way to tell? I failed to find the study on PubMed and found only the PDF of the poster presented at a conference, which fails to clarify the issue.

Solution 1:

In your quote from MDmag, I suppose they are discriminating from drug use by means of snorting, smoking or suppository application, none of which will be relevant in the context of HIV transference. In context, the statement seems to mean illicit narcotics. It is unlikely to refer to, for example, diabetic persons injecting insulin, though it could do.

AidsInfo uses the term 'injection drug use' - again, in context - to mean the injection of illicit drugs.

A method of illicit drug use. The drugs are injected directly into the body

Solution 2:

I expect that the term "use" implies "non-prescription" -- that phrases like "Are you a drug-user?" and "Have you ever used drugs?" would IMO never be used to refer to prescribed medications.

The linked PDF mentions "substance use disorder" which implies that meaning of the word "use".

Also I'd expect that clinical injections needn't correlate with blood-borne disease like HIV, whereas injection drug abuse is correlated because (so I've read) people unsafely share and reuse needles.

I think you're right that it is (or might seem) ambiguous or ill-defined, but I'd assume it means (or I think it implies) illegal drug abuse ... so much so that if they had meant to include "therapeutic" they'd have said so explicitly.