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).
- it gave awful hallucinations, probably preventing he who drank it to answer correctly questions
- 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?