Hi,
It depends on the drug, the preference of the vet to some extent, and maybe the injection volume.
Intravenous injection (rare, only used generally in critical situations) would basically be in the ventral tail vein.
Intracoelomic dosing (as described in the article linked) may be used but risks laceration of internal organs, as well as being subject to renal portal system effects (see below)
Intramuscular could be in the limbs or epaxial muscles (round the spine). Muscle damage is likely, depending a bit on drug and dose
Subcutaneous dosing is theoretically not possible on caudates or caecilians; they have effectively no subcut space. Subcutaneous lymph sacs exist however and may be dosed by a basically similar method
Topical (transcutaneous) dosing is feasible and considered generally effective in amphibians (with a suitable drug/volume
The tail/rear half of body thing is because amphibians have a renal portal system - basically the blood from the rear half of the body flows through the kidneys before going back through the heart and into general circulation. The theory is that this could either a) risk kidney toxicity if it's that type of drug or b) be cleared significantly from the blood before becoming effective (if it's kidney-excreted). The limited evidence we have in herps generally suggest that it doesn't actually make a significant difference in practice but it is still a risk - unless there is a very good reason for injecting in the back half, best to avoid it.
Hope this helps,
Bruce.
If a doctor were to give an antibiotic shot to a salamander/newt, where would the injection site be? Anyone ever seen a doctor give one?