Chemicals in plants – cyanide

Hydrogen cyanide (aka hydrocyanic acid) has a faint bitter-almond scent – although not everyone can smell it, due to a recessive gene. It is released in plants by compounds called cyanogenic glycosides. One is a cyanohydrin called amygdalin. Amygdalin will slowly release hydrogen cyanide when it comes into contact with the plant enzymes that are activated by tissue damage, caused by chewing or crushing. Fruits in the Rosaceae family, such as apples, apricots, bitter almonds, cherries, crab apples, damsons, hawthorn berries, pears, peaches, plums and sloes, have stones that contain small amounts of amygdalin, and other cyanohydrins like mandelonitrile. One hundred grams of crushed apple seeds can yield about 70 mg of hydrogen cyanide. 

The lowest reported oral lethal dose of hydrogen cyanide for humans is 0.54 mg/kg body weight (WHO report), and the average absorbed dose at the time of death has been estimated at 1.4 mg hydrogen cyanide per kilo of body weight. So, if you weigh 70 kg, your lowest lethal dose is 37.8 mg – about 54 grams of crushed apple seeds! You need to crush them – ideally into a powder – for all the enzymes to come into contact with all the amygdalin. If you’re worried about this, as symptoms of toxicity can occur from hydrogen cyanide at a concentration of 0.5 mg/kg of body weight, and some apples contain up to 4 mg amygdalin per gram, assuming an apple pip weighs about 0.7 g, avoid powdering and eating 66 apple seeds in one go. Incidentally, the amygdalin content of commercially-available pressed apple juice is low, ranging from 0.01 to 0.04 mg per ml.

Other popular farmed crops that are highly cyanogenic include lima beans, butter beans, almond, sorghum, macadamia nut, flax, white clover and cassava. (The “bitter” roots of the cassava plant may yield up to 1 gram of hydrogen cyanide per kilogram.) Seeds from cucumber, courgette, marrow, melon and squash also contain low levels of amygdalin – under a quarter of a gram per gram of seed. Processing, such as toasting pumpkin seeds and UHT processing of apple juice, also lowers the levels still further. 

Incidentally, it’s not just plants that do this. Your own neurons can also release hydrocyanic acid when your opioid receptors are activated. This is necessary for opioids to provide pain relief. Your blood cells (leukocytes) also generate hydrocyanic acid when they are engulfing other cells (phagocytosis). They can kill bacteria, fungi, and other pathogens by generating several different toxic chemicals, one of which is hydrogen cyanide. You’re most likely to come into contact with hydrogen cyanide from vehicle exhaust fumes, cigarette smoke and the smoke from burning plastics. 

Amygdalin decomposes into three parts. Hydrogen cyanide (as mentioned), glucose and benzaldehyde. Benzaldehyde, which tastes strongly of almonds, amaretto or marzipan is a common chemical in many young plants. Rowan buds for example, have a very intense marzipan flavour. Blackthorn (sloe) leaves are used to make a drink called épine which tastes like an amaretto-flavoured port. The flowers of hawthorn, sloe and cherry all smell of benzaldehyde.

Hydrogen cyanide is released to put mammals and insects off munching on the plant’s seeds. Cyanide-free benzaldehyde is an attractive flower scent for insect pollinators. 

Footnote on cherry brandy
Having been asked about cherry pit brandy, most commercial brandies like Bols are made from pitted cherries with just a few stones added for taste. People who have made cherry brandy keeping the stones in the cherries, for a year or two, have reported headaches and high blood pressure. However, there is a lot of debate about this. Cracking the stones and kernels releases more cyanide as it triggers the enzyme reaction, as does crushing the kernels and people’s individual tolerance for eating apple pips and cherry pips varies widely. Some react to a single pit, some have eaten them for years! A calculation done by D S O’Neil here, works out that ‪500mg pits in 1L brandy yields about 1mg HCN per ml. So 50ml – a double shot – could be very unpleasant and even kill a weak or sick person. O’Neil doesn’t state if this is cracked kernels but I assume it is cracked and ground, as in the apple pip illustration above, with all the amygdalin activated and converted. But to cut a long story short… keep pits a LOT lower than this ratio to be on the safe side and just leave them in for a few weeks and not years! 


  1. Thanks! That’s very interesting. I grew up eating loads of dried apricot kernels – great fun cracking them with a rock and popping straight into your mouth. Does drying them decreases the amount of HCN? I still eat them occasionally and always add a few to my apricot jam – wouldn’t taste the same without them!

    • If you’re making jam then you’re cooking the kernels at quite a high temperature. As dried kernels can last years before growing again when watered, they can still activate the enzymes need to create HCN. Commercially bought kernels are often dried at high temperatures.

  2. abuHamzah albouriny

    Thank u very much.
    so is it true that soaked kernels have more hcn than dried ones?
    i mean if eaten dried will these kernels be less hcn activated than if they where soaked?

    thank u very much.

  3. Justine Johnstone

    Thank you. This is really interesting. I have some vodka that has had sloes steeping away in it for 2-3 years. Do you think there is any risk of poisoning from the stones (not cracked)? Should I remove the fruit just to be on the safe side?

    • The fruit doesn’t need to be in there for years. After 2-3 months the flavour will have been extracted by the alcohol. So they should be strained off, bottled and hidden away to mature. Prolonged soaking of plants in general tends to result in higher levels of tannins, alkaloids, bitterness. I doubt your sloe vodka is poisonous but it could be mouth-puckeringly tannic and have a hangover-inducing. But perhaps not. Strain them off now and see what it’s like. Add a touch more sugar if needed and hide it at the back of the cupboard for a few more years!

  4. jim harris

    hello Monica
    Is it safe to do tree Huggin in the forest ie a Uk European forest for example
    do trees such as oak or silver birch etc release any chemicals such as hydrogen cyanide as a defensive mechanism when you hug them
    asking this because I once hugged a tree for about 2 mins and later the next day and a few days afterwards
    I had a strange sensation in my nose that I can only describe as felt like I could taste a cold log taste in my nose and back of my mouth

    so im worried something was released from the tree that got into me at the time

    can you help about this in anyway
    thanks from jim harris in the uk

    • I hug trees all the time. No they don’t emit hydrogen cyanide in self defense. Pine trees release phytoncides during peaceful days and these are beneficial for us. They reduce high blood pressure, high heart rate and make us feel calm. Birch trees only emit methyl salicylate if one has been attacked or chopped down. This is an aspirin-like compound we call ‘wintergreen’ that is used in rubs to relieve arthritic pain. It tastes a little medicinal but I have only tasted it from chewing a birch leaf and in roasted birch twig tea, but never from hugging a tree. I advise hugging oak, beech, birch, rowan, hazel, chestnut (both types), sycamore, pine, larch, giant redwoods. I treat yew trees with respect and just give them a pat – they have poisonous leaves and bark. If you’re not confident perhaps go on a guided shinri yoku or forest bathing walk. My daughter’s website explains it in more detail

  5. Monica, I’m hunting information about the half-life of cyanide as it composts out of cassava root bark waste.
    When the roots are processed for eating, there is at least one layer, sometimes two layers removed from the root prior to cooking.
    All that waste raw vegetable may be composted but does the cyanide build up in the compost or soil over time?
    I know that heat or cooking destroys cyanide but what happens to the chemical from uncooked vegetable?
    Also does cyanide kill earthworms that process the compost?

  6. Sue Shapcott

    This is brilliant. I have been told there is cyanide in sloes and that I shouldn’t eat the fruit, but now I know to avoid the stones/pips and to heat the fruit with sugar to reduce any cyanide and to take away the bitter taste. I had no idea before about there being cyanide in so many fruits and vegetables that I have eaten since childhood. However, I have suffered from migraine and headache, so maybe that is why. I used to eat the whole apple, pips and all!

    • If you’ve suffered migraines it can be a variety of reasons including that your blood is out of kilter and on the thick side. Can recommend seeing a qualified herbalist.

  7. Hi Monica,
    This comment is very late but I was wondering about the amygdalin content in hawthorn berry seeds. I can’t seem to find out anywhere whether they contain large amounts than apple seeds! Could you help me out?

    • I don’t know. I personally don’t eat the seeds raw so it’s not relevant. If roasting them as a coffee substitute the compounds are destroyed by the heat.

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