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The legal question is whether Sam's punch can be treated as having caused Reece's death, given the intervening negligent administration of an allergen by hospital staff. Causation in English criminal law is a two-stage test: factual causation, then legal causation.

Factual causation applies the "but for" test (R v White — defendant put cyanide in his mother's drink but she died of an unrelated heart attack first; factual causation failed because her death did not depend on his act).

Legal causation requires the defendant's conduct to be (i) more than a minimal contribution (R v Kimsey: more than a "slight or trifling link"), and (ii) an operating and substantial cause at the time of death (R v Smith, where the stabbed soldier was dropped three times en route to the medical station and received bad treatment, but his original wound remained the operating cause).

Medical novus actus interveniens (NAI) sets a high threshold. R v Cheshire establishes that medical treatment will not break the chain unless it is so independent of the defendant's act and so potent in causing death that the defendant's contribution becomes insignificant. R v Jordan is the rare counter-example: the deceased's wounds had "mainly healed" and the treatment was found to be "palpably bad" — the original injury was no longer operating.

Factual causation: but for Sam's punch, Reece would not have been knocked unconscious, would not have been admitted to hospital, would not have been administered the painkiller, and would not have died. Factual causation is comfortably satisfied — distinguishable from White because death here flows directly from the chain Sam set in motion.

Legal causation: Sam's punch caused unconsciousness severe enough to require hospitalisation; without it the medical situation would never have arisen. The contribution is plainly more than the minimal Kimsey threshold. As in Smith, the original injury remained operating: Reece had not recovered when the painkiller was given.

Medical NAI: administering a known allergen to a patient with a documented allergy is serious medical conduct, closer in character to Jordan ("palpably bad" treatment) than to the more routine errors in Cheshire. However, the better view is that this still does not reach the Jordan threshold. Unlike Jordan, Reece's injuries had not "mainly healed" — the unconsciousness from blunt-force trauma was still operating at the time the painkiller was administered. A reasonable jury could go either way: the prosecution will argue Cheshire applies because Sam's act created the situation; the defence will argue Jordan applies because the medical conduct was radically unwarranted and independently fatal.

Factual causation is satisfied. Legal causation is more finely balanced, but on the better view the chain is preserved: Sam's punch remained an operating and substantial cause, and the medical error — though serious — did not reach the Jordan threshold of treatment so radically unwarranted that the original injury became insignificant. Sam can therefore be held to have caused Reece's death, with the qualification that a defence of medical NAI under Jordan is at least arguable.