Is it against any law to inject heroin in Antarctica?
The short answer would seem to be its a mess.
Skip down to the bit about the potential murder there, and how its caused real problems jurisdiction wise.
Otara
You might get in trouble if you inject it into penguins…
Yeah, the laws of physics. A hypodermic needle will not penetrate ice. 
A more serious answer: The political status of Antarctica and adjacent islands is governed by the U.N. Antarctic Treaty. However, it’s my understanding that within individual stations and their surroundings, the relevant laws of the member parties with research stations, tourism, etc., there, are applicable. In other words, something happening at or around McMurdo would be governed by U.S. law (U.C.M.J., it being a naval base); at or around Vostok, by Russian law; and the same for Chilean, Norwegian, Argentine, etc., bases.
Is every inch of Antarctica governed by a research station? What if it wasn’t in a research station?
Then you’re standing in the middle of an Antarctic autumn watching the march of the penguins, needle stuck in your arm, and dopey grin on your face. I still think you would get busted since you have to travel through one of these stations anyways to get out onto the wastelands of Antarctica. You can’t just fly a plane onto a random ice shelf, smoke your tar and enjoy.
…Need Answer Fast!
Some countries claim that their laws, or certain of their laws, apply to their citizens extraterritorially. So even if you were in some region of Antarctica unclaimed by a particular jurisdiction, you might be arrested upon return to your home jurisdiction, provided the crime was notorious enough.
That said, I can’t think of any law which prohibits injecting heroin into yourself. Drug laws are almost always worded in such a way as to criminalize ownership or sale of drugs, not mere personal use. On the other hand, if you were planning on injecting heroin into someone else without their consent, then you might run afoul of laws against poisoning.
Sure you can. There are no border and custom controls there; you can land your plane or your ship wherever you want without first visiting a research station.
The problem might be at your port of departure, though. Your personal belongings and cargo may well be inspected at the airport/seaport before you’re cleared to leave.
Would not possession, rather than ownership, be the offense? (That would be partly because it’s easier to prove, and partly so you can’t argue, “I’m just holding it for this stranger, who mysteriously disappeared.”) If it’s an offense to possess heroin, it’s hard to conceive how you could inject it into yourself without possessing it.
Well, if someone else was holding a syringe full of heroin over your arm, presumably they would be the possessor, and your pressing down the plunger would be the act of injecting it into yourself. So it’s fairly convoluted, but hypothetically possible to inject without possessing. Whether a court would see it that way I suppose depends on how the law is written and how the judge and/or jury elect to interpret the law and the evidence.
Or a zealous DA would say that once it was in your vein, you’re possessing it. Now, I know ridiculous arguments are never taken seriously when OMG DRUGS! are involved, but there’s always a first time…