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When discovering the locket's basin, Harry thinks the potion will kill whoever drinks it, but Dumbledore thinks otherwise:

"I can only conclude that this potion is supposed to be drunk."

"What?" said Harry. "No!"

"Yes, I think so: only by drinking it can I empty the basin and see what lies in its depths."

"But what if-- what if it kills you?"

"Oh, I doubt that it would work like that," said Dumbledore easily. "Lord Voldemort would not want to kill the person who reached this island."

Harry couldn't believe it. Was this more of Dumbledore's insane determination to see good in everyone?

"Sir," said Harry, trying to keep his voice reasonable, "sir, this is Voldemort we're --"

"I'm sorry, Harry; I should have said, he would not want to immediately kill the person who reached this island," Dumbledore corrected himself. "He would want to keep them alive long enough to find out how they managed to penetrate so far through his defenses and, most importantly of all, why they were so intent upon emptying the basin. Do not forget that Lord Voldemort believes that he alone knows about his Horcruxes."
(Half-Blood Prince, Chapter 21, The Cave)

I think we can accept Dumbledore's reasoning: Voldemort would have want to question the person who found the cave, not only to know how, but also to know if (s)he told someone else about it...

And, indeed, the potion was not instantaneously lethal (the question whether it was lethal in itself with more time is discussed here).

BUT:
- it gave awful hallucinations, probably preventing he who drank it to answer correctly questions

AND
- it provoked a huge thirst
- it was in a place where no water could be drunk... except the one available in a lake full of Inferi that tried to drag you to their world as soon as you touched the water.

So, all in all, the whole system was lethal, not instantanesouly, but, in, let's say, less than one hour.

So Voldemort would have had to come to the cave in double quick time to get a chance to question the intruder. But we know he doesn't have any presense sensor, otherwise he wouldn't have wait several months (for Harry and Dumbledore's visit) or years (for Regulus' one) to check if someone had visited his cave:

True, he had not felt it when the diary had been destroyed, but he had thought that was because he had no body to fell, being less than ghost... No, surely, the rest were safe... The other Horcruxes must be intact...
But he must know, he must be sure... He paced the room, kicking aside the goblin’s corpse as he passed, and the pictures blurred and burned in his boiling brain: the lake, the shack, and Hogwarts— [...] And how could the boy, or anybody else, know about the cave or penetrate its protection? The idea of the locket being stolen was absurd...
(Deathly Hallows, Chapter 27, The final hiding place)

So how did he intend to question the person who would access to the cave?

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    i feel like your last quote is the best thing we have to go off of. – Himarm Apr 22 '16 at 15:03
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    I wonder if it's to do with the degradation of his soul connections? Maybe the locket was supposed to "call out" to him if the liquid was drunk, but as he made more Horcuxes he became unable to "hear" it? – DavidS Apr 22 '16 at 15:20
  • @DavidS That is actually very good reasoning. Why don't you add that as an answer along with quotes from canon (e.g. where Harry asks Dumbledore in HBP whether Voldemort can feel it if a horcrux has been destroyed)? – rah4927 Apr 22 '16 at 19:07
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    It's possible that Dumbledore was simply mistaken about that. – Harry Johnston Apr 23 '16 at 14:33
  • @HarryJohnston Yes, maybe that's just this... Nothing else is coherent enough with everything... but I find hard to accept this: alright, he makes mistakes too, but this reasoning is so logical! – LilyM Apr 26 '16 at 13:08
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Maybe he didn't...

...at least, not if they actually got the locket.

It seems clear to me that the potion is meant to kill the person who took it, if they drink all of it. When Regulus Black drank all the potion, he was dragged beneath the water and killed.

“And he ordered—Kreacher to leave—without him. And he told Kreacher—to go home—and never to tell my Mistress—what he had done—but to destroy— the first locket. And he drank—all the potion—and Kreacher swapped the lockets—and watched . . . as Master Regulus . . . was dragged beneath the water . . . and . . . “

The Inferi clearly are not taking any prisoners. But this makes sense: once someone has drunk all the potion, they have access to the locket, a piece of Voldemort's precious soul. Would the desire to acquire information really supersede the safety of that artifact, especially for someone like Voldemort?

Rather, the potion does not immediately kill someone if they do not drink all of it, which anyone not obsessed with getting the Horcruxes would do, since it is incredibly painful and traumatic.

It seems likely, then, that once one has drunk some of the potion, but not all of it, leaving the island is made impossible by some method, and perhaps a signal sent to Voldemort.

Of course, this would not have happened in the case that all the potion was drunk, because Voldemort knew that a single wizard would inevitably die if they did such a thing, and that he could acquire little further information if that happened. Again, keep in mind that killing those who actually have access to the locket is probably a good tradeoff.

As usual, though, he underestimated the talents of house-elves and children.

Note: @DavidS also had a good suggestion. Maybe he believed that he would know as soon as anyone tried to steal a Horcrux. He certainly believed that he would know if they were destroyed:

But surely if the boy had destroyed any of his Horcruxes, he, Lord Voldemort, would have known, would have felt it? He, the greatest wizard of them all; he, the most powerful; he, the killer of Dumbledore and of how many other worthless, nameless men. How could Lord Voldemort not have known, if he, himself, most important and precious, had been attacked, mutilated?

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I have to preface this by stating that this is simply my opinion... but I've read the books fully through enough to have a decent sense of what's going on in the characters' heads.

We know that Dumbledore was already regaining his mental whereabouts when they returned to the castle. Had they not been ambushed, I think it's safe to say he would have naturally recovered.

With that in mind...

The potion is designed to disorient and mentally incapacitate the drinker, which would make it easy for Voldemort to restrain them for questioning when the potion wore off. It would also give him time to get there while they were in that state.

Could they have easily died by entering the water or by some other means before he arrived? Sure but Voldemort would find that of little concern. He would want to know how they found the Horcrux but not leave the ability for them to escape in tact.

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    The problem with this is that he almost never visits the cave, and he seems to have no 'Intruder Alert' feature. Given that he wouldn't know he had an intruder to interrogate, they would either die from starvation/trying to drink the water or would recover and leave the cave. The protection is either die or escape, the interrogation part make no sense unless there was an 'Intruder Alert' – Matt Sep 1 '17 at 15:48

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