Zucchini may occasionally contain a group of natural toxins known as cucurbitacins. These toxins give zucchini a bitter taste. Bitterness in wild zucchinis has been known for a long time but is rarely found in commercially grown zucchinis.
Eating bitter zucchinis have caused people to experience vomiting, stomach cramps, diarrhoea and collapse. Do not eat zucchini that have a strong unpleasant smell or taste bitter. See more about Natural toxins in food