Description
Amygdalin is a naturally occurring plant compound belonging to the group of cyanogenic glycosides characterized by the release of cyanide upon enzymatic degradation. It is primarily found in the seeds of apricots, peaches and bitter almonds and also in plants such as lima beans, clover and sorghum.
Laetrile is an acronym (derived from LAEvorotatory and mandeloniTRILE) used to describe a purified, semi-synthetic form of amygdalin. Laetrile is claimed to be an anti-cancer treatment or even cure, and cyanide is thought to be the active anti-cancer ingredient.
It was at the height of its popularity in the U.S. in the 1970s and 1980s until the FDA considered it unsafe after a phase I and phase II controlled clinical trial in 1982 found no evidence for the efficacy of laetrile and highlighted considerable safety concerns including cyanide poisoning.
Efficacy
There is no evidence available for the effectiveness of drugs containing amygdalin in antitumor treatment.
Safety
Amygdalin has been associated with significant risks including cyanide poisoning.
Citation
Mora D, Wider B, CAM Cancer Collaboration. Amygdalin/Laetrile [online document], Aug 31, 2023.
Document history
Assessed as up to date in August 2023 by Dana Mora. Assessed as up to date and revised in June 2021, August 2020, January 2019 and February 2017 by Barbara Wider. Updated and revised in July 2015 by Adele Stapf. Updated in April 2014 by Barbara Wider. Assessed as up to date in December 2012 by Helen Cooke. Updated and revised in December 2011 and July 2009 by Helen Cooke. Summary first published in September 2005, authored by Helen Seers. Next update due: Aug, 2026
Description and background
The terms amygdalin, laetrile and “vitamin B17” are often used synonymously but refer to different products.
Amygdalin is a secondary plant compound belonging to the group of cyanogenic glycosides characterized by the release of cyanide upon enzymatic degradation. Amygdalin is also known as mandelonitrile-beta-glucuronide (semi-synthetic), mandelonitrile beta-D-gentiobioside (natural product), amygdalina, and nitriloside.
Laetrile is an acronym derived from ‘LAEvorotatory’ and ‘mandeloniTRILE’ and is used to describe a purified, semi-synthetic form of amygdalin.
Although not a vitamin, amygdalin is sometimes also referred to as “vitamin B17”, a name given to it by chemist E.T. Krebs. According to a historical theory cancer develops as a result of deficiency of "vitamin B17" (Milazzo 2015).
Amygdalin was first identified and isolated by French chemists in 1830 and was used as an anti-cancer agent in 1845 in Russia. By the 1920s amygdalin had reached the US, but the early pill form was considered to be too toxic and its use was discontinued. In the 1950s an apparently non-toxic, semi--synthetic intravenous form of amygdalin was developed by Ernst T Krebs in the U.S., and this became known as laetrile.
In the 1970s laetrile gained popularity either as a single-agent treatment or as part of a metabolic treatment regimen that also included high-dose vitamin supplements and enzymes (Chandler 1984). The popularity of laetrile reached its peak in the United States in 1978 when it was reported that 70,000 people had been treated with it (Chandler 1984). Current prevalence data are not available.
Ingredients and quality issues
Amygdalin is a secondary plant compound belonging to the group of cyanogenic glycosides, which are enzymatically degraded by beta-glucosidases to form cyanide. The compound is found in the seeds of apricots, peaches and bitter almonds and also in plants such as lima beans, clover and sorghum. Minor amounts of amygdalin may be ingested when eating a balanced diet.
The chemistry of laetrile available in the United States is different from the one used in Mexico. The laetrile used in Mexico may be crushed apricot seeds rather than the semi-synthetic form of laetrile (mandelonitrile-beta-glucuronide) that is used in the US (Davignon 1978). Therefore it is conceivable that not all studies on laetrile have looked at the same compound.
Application and dosage
Amygdalin is usually taken orally in the form of bitter apricot seeds. Laetrile is available for oral administration as well as intravenous or intramuscular injection. Treatment is typically initiated intravenously for two to three weeks and then continued orally. Laetrile is also used as an enema form or applied directly to skin lesions.
Alleged indications
Suppliers have postulated the use of amygdalin/laetrile as anti-cancer treatment. Many supporters of laetrile refer to anthropological evidence from epidemiological studies of secluded cultures consuming high levels of foods rich in amygdalin, e.g. the Hunzakots of Pakistan, the Inuit of the Arctic or the indigenous Hopi and Navajo of North America (McCarrison 1936).
Expeditions have reported extraordinary longevity and the absence of cancer in these cultures, however the studies are often flawed due to language barriers, highly selected contacts and sometimes deliberate misinformation. They fail to provide clear evidence for a causal relationship between the uptake of amygdalin and the incidence of cancer.
Mechanisms of action
It has been claimed that amygdalin could exert its anticancer activity via inducing apoptosis, inhibiting tumour cell growth, and reducing tumour cell metastasis (He 2020).
Proponents ascribe the anticancer effects to the cyanide released upon enzymatic degradation of the compound. They postulate that amygdalin is selectively effective against cancer cells only. This hypothesis involves an imbalance of the enzymes beta-glucosidases, beta-glucuronidase and rhodanase in cancer cells as compared to healthy cells.
Malignant cells are claimed to be specifically vulnerable to cyanogenic glycosides because of two characteristics: firstly, a higher level of beta-glucosidases and beta-glucuronidase compared to healthy cells, which would lead to a more rapid intracellular release of cyanide from laetrile or amygdalin, and secondly, a deficiency in rhodanase, an enzyme that converts cyanide into the harmless compound thiocyanate (Ellison 1978, BfArM 2014, He 2020).
Legal issues
In the US laetrile has had a long controversial history. This includes inaccurate theories of how it works, conspiracy theories of unpublished research studies supporting its use, banning of its use in the US by the Food and Drug Administration (FDA), and prison sentences for many proponents and suppliers (including ET Krebs himself). The controversy continues.
In the United Kingdom the Medicines and Healthcare Products Regulatory Agency (MHRA) has assigned amygdalin/laetrile /B17 the status of a prescription-only medicine. The substance is not banned but it is unlicensed, and so access to it is subject to prescription by a medical doctor. This implies that doctors can supply it to a patient should they consider it an appropriate treatment, but do so at their own risk. There is no licensed product containing the substance laetrile/amygdalin/B17 in the UK.
Drugs containing amygdalin are not approved by the regulatory authorities in Germany and are considered unsafe according to the German Drug Act.
No randomized clinical trials of laetrile/amygdalin are available. Only one small phase I trial (n=6) and a phase II trial (n=175) are available.
Description of included studies
Antitumour treatment
The authors of Cochrane systematic review published in 2015 and declared as stable in 2018 were unable to find any randomised controlled trials assessing the effectiveness of laetrile. They concluded that there was no evidence that laetrile was effective as an anti-cancer agent (Milazzo 2015). Two clinical trials have been conducted in the early 1980s using amygdalin/laetrile.
In the first uncontrolled study (phase I; n = 6) amygdalin was applied intravenously at a dosage of 4.5 g/m2 body surface followed by oral administration of 0.5 g three times daily (Moertel 1981). Additionally, patients took a combination of several vitamins and enzymes and followed a diet. Two of the six tumour patients studied developed symptoms of cyanide poisoning while they were taking oral amygdalin. No indications of clinical effectiveness were found.
The second clinical trial (phase II; n = 175) compared the above amygdalin dosage regimen versus a “high-dose group” (7 g/m2 body surface i.v. for 21 days followed by 0.5 g four times daily). Also the adjunct treatment programme comprising vitamins, dietary guidelines and enzymes differed in terms of the dosages used. According to the authors, no substantive benefit was observed in terms of cure, improvement or stabilization of cancer, improvement of symptoms related to cancer, or extension of life span. The hazards of amygdalin therapy were evidenced in several patients by symptoms of cyanide toxicity or by blood cyanide levels approaching the lethal range (Moertel 1082).
Adverse events
Since the tolerance of amygdalin may vary greatly, it is impossible to predict the risk for an individual patient. A review concluded that there is a high risk of cyanide poisoning when amygdalin is taken orally. The intestinal microflora contains enzymes that enhance the release of cyanide in the intestinal tract (Newmark 1981). Studies revealed that amygdalin's toxicity was caused by its poisonous decomposite product of benzaldehyde and hydrogen cyanide after oral ingestion, toxicity of intravenous administration route was far less than the oral route. (Milazzo 2015, He 2020).
The adverse events observed reflect the symptoms of cyanide poisoning. Headache, dizziness, nausea and vomiting, dermatitis or, in severe cases, disturbed consciousness, tachycardia, respiratory distress, liver damage, coma and death may occur following oral administration. Several fatalities were attributed to drug preparations containing amygdalin (Braico 1979, Sadoff 1978).
The risk of cyanide poisoning with oral intake of laetrile is increased when vitamin C is taken additionally (Lee 1982). A case of adverse drug reaction showing signs and symptoms of a serious, life-threatening inadvertent cyanide poisoning has been reported in Australia in 2005. The report suggests that a female patient increased the cyanide toxicity risk by taking amygdalin concomitantly with a high dose of vitamin C (4,800 mg) (Bromley 2005). A case report of a 4-year-old boy with severe encephalopathy due to cyanide poisoning after oral and intravenous administration of amygdalin has been published in 2015 (Sauer 2015).
In 2022, a case report was published of a 72-year-old man with pancreatic and mediastinal cancer who suffered cyanide intoxication after ingesting 2g of an amygdalin supplement (Cmorej 2022).
A 2019 study in 55 tumour patients assessed the cyanide blood levels after chronic oral amygdalin intake and subsequent intravenous amygdalin administration. Participants received 3 x 500 mg/day of oral amygdalin as well as 1-5 i.v. amygdalin treatments with 6-18mg every 4-6 days. The mean cyanide blood levels increased significantly from pre- to post i.v. amygdalin administration (34.74 μg/L to 66.20 μg/L). (Manos 2019)
Amygdalin preparations contaminated with beta-glucosidases, seen particularly when the product is made from apricot seeds, may enhance the hydrolysis of amygdalin and thus considerably increase its toxicity. In the presence of such contaminants, parenteral application of amygdalin may cause cyanide poisoning (BfArM 2014).
Contraindications
In addition to the risks mentioned above, products containing amygdalin may decrease liver function in patients with hepatic impairment (Sadoff 1978). The use of amygdalin during pregnancy or breast-feeding is contraindicated.
Interactions
An experimental animal study showed no effect of amygdalin on the activity of the CYP2B isoenzymes; however there may be an effect on the subgroups CYP2A, 2C and 3A. There are no indications of any clinically relevant pharmacokinetic interactions (Yamada 1998).
Warnings
Particularly oral ingestion may result in a dose-dependent severe, possibly fatal cyanide poisoning.
Additionally, cases of mislabelling have been observed and investigations of samples showed contamination with bacteria, toxins and other substances.
BfArM. Bulletin zur Arzneimittelsicherheit, Informationen aus BfArM und PEI: Amygdalin. Ausgabe 3, September 2014. Available online, accessed 28th September 2023.
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