Why did Lestat stop Claudia from drinking before the victim's heart stopped beating?

Vampires drink blood because it sustains them. In the Ricean universe, a vampire must not drink blood from a dead person. "Dead blood" will make them seriously ill.

And confirmed by Lestat's own storyline IN the film, it does NOT kill him as some mistakenly think. The uncirculating blood becomes toxic and therefore, the vampire must stop drinking before the heart stops beating, even if there is more blood left in the victim's veins. Lestat is forced to drink from a corpse in the novel's sequel and all it causes is what he calls a temporary "rolling delirium."

It's theorized that this dizziness is the result of the blood not gaining fresh oxygen through circulation. More.

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