Mushroom poisoning (also known as mycetism and mycetismus) occurs after consuming a toxic mushroom. Only 5% of all known mushroom species are poisonous, although new toxic mushroom species are continually identified. Misidentification of mushrooms is common. Mushroom poisoning presents differently based on the toxin consumed. Severity of the symptoms ranges from mild to deadly. Most fatal ingestions are due to consumption of Amanita phalloides.

The exact prevalence of mushroom poisoning fatalities is unknown but is thought to occur in the 100s each year. An increasing risk is emerging in parts of the world with a large influx of migrants, some of whom forage for food due to poor economic circumstances.

A key consideration in classifying type of mushroom poisoning has traditionally been time of onset of gastrointestinal (GI) symptoms. Generally, but not universally, a latency of less than 6 hours portends limited toxicity. Conversely, if GI symptoms occur more than 6 hours after ingestion, a more serious clinical course should be considered. However, as more poisonous mushrooms continue to be discovered, this timeline paradigm continues to be modified and is becoming less of a reliable guideline for prognostication.