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The phrase "Warburg effect" is used for two unrelated observations in biochemistry, one in plant physiology and the other in oncology, both due to Nobel laureate Otto Heinrich Warburg.

In plant physiology, the Warburg effect is the inhibition of carbon dioxide fixation, and subsequently of photosynthesis, by high oxygen concentrations. The oxygenase activity of RuBisCO, which initiates the process of photorespiration, largely accounts for this effect.

In oncology, the Warburg effect is the observation that most cancer cells predominantly produce energy by glycolysis followed by lactic acid fermentation in the cytosol, rather than by oxidation of pyruvate in mitochondria like most normal cells. This occurs even if oxygen is plentiful. Otto Warburg postulated that this change in metabolism is the fundamental cause of cancer, a claim now known as the Warburg hypothesis. Today it is thought that genetic mutations are the fundamental cause of cancer. The Warburg effect may simply be a consequence of damage to the mitochondria in cancer, or an adaptation to low-oxygen environments within tumors, or a result of cancer genes shutting down the mitochondria because they are involved in the cell's apoptosis program which would otherwise kill cancerous cells.

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