Almonds and Bitter Almonds
Almonds (sweet) are used in several cocktail ingredients, including orgeat syrup and falernum syrup/liqueur. Sweet almonds are legal for food use in the United States.
Bitter almonds, which provide the flavoring for amaretto liqueurs, are "generally recognized as safe" in the US only if they are "free from prussic acid." [link] Often amarettos are produced with flavoring not from actual bitter almonds but from the kernels of peaches, cherries, and/or apricots, which also contain prussic acid (hydrogen cyanide). Recipes appear online for homemade amarettos with unsafe quantities of bitter almonds or other kernels.
Some people are allergic to almonds, so bartenders should consider making the ingredient explicit on cocktail menus rather than saying "orgeat".
Almonds are seeds and not true nuts, [link] yet some people with or without nut allergies are allergic to almonds.
Both sweet almonds and bitter almonds contain substances that convert into the poison cyanide, but bitter almonds contain about 42 times the amount of hydrocyanic acid (HCN) [link]. For simplicity's sake, this post will refer cyanide and not a "cyanide precursor."
Though bitter almonds are a species of almonds, for commercial amarettos the kernels of peaches, cherries, and/or apricots are typically used instead. In the production of bitter almond flavoring (accomplished with a water distillation at at least one company [link]), the cyanide is separated or eliminated so that it is not in the final product.
In the United States, bitter almonds must be "free from prussic acid" (hydrogen cyanide) [link].
Other ingredients that are legally required to have limited amounts of prussic acid according to the US TTB are: cherry pits, cherry-laurel leaves, elder tree leaves, and peach leaves. These are limited to "25 ppm prussic acid in the flavor" [link].
According to this scientific paper [link], the maximum permitted HCN levels in drinks in other countries is:
Australia, New Zealand Food Standards code is 5 mg/kg
But in the European Union, the limits (via this 2004 article) are:
Current EU regulatory status: Annex II of Directive 88/388/EEC on flavourings sets the following maximum levels for hydrocyanic acid in foodstuffs and beverages to which flavourings or other food ingredients with flavouring properties have been added: 1 mg/kg in foodstuffs, 1 mg/kg in beverages, with the exception of 50 mg/kg in nougat, marzipan or its substitutes or similar products, 1 mg per percent volume of alcohol in alcoholic beverages and 5 mg/kg in canned stone fruit. Hydrocyanic acid may not be added as such to foodstuffs (EEC, 1988).
In Canada, "Almond Essence, Almond Extract or Almond Flavour shall be the essence, extract or flavour derived from the kernels of the bitter almond, apricot or peach and shall contain not less than one per cent by volume of hydrocyanic acid-free volatile oil obtained therefrom." [link]
Canada has proposed updating the law to apricot kernels to ensure ones used directly or intended for further processing contains safe amounts of cyanide, according to this document from December 2018 [link].
The cyanide coming from bitter almonds is a well-known poison. "High exposure to this potent poison in humans may cause nausea, vomiting, diarrhea, dizziness, weakness, mental confusion, and convulsions followed by terminal coma and literally death." [link]
According to one study [link], "the acute oral lethal dose of HCN for humans is reported to be 0.5–3.5 mg/kg of body weight and the consumption of 50 bitter almonds is deadly for adults. However, for young children, 5–10 almonds are fatal."
The European Food Safety Authority writes, "Eating more than three small raw apricot kernels, or less than half of one large kernel, in a serving can exceed safe levels." [link]
For apricot kernels, "severe toxicity would be inevitable due to the consumption of approximately 30 apricot kernels for adults and fewer for children." [link] The same study cites a recommendation from another source that "Consumers are advised to eat only five kernels in one hour and no more than 10 per day."
We cannot expect every customer to know that orgeat is an almond syrup, nor that falernum may contain almonds. Consider explicitly listing these ingredients and others that contain almonds as "almond syrup" for example, or provide other warning on the menu.
Peach Tree Leaves
There has been an inquiry about peach tree leaf toxicity. According to the University of California [link]: "All parts of the peach tree, with the exception of the edible peach fruit, contain cyanide-producing compounds that are released when peach seeds, bark, and leaves are eaten. Children have died from eating the seeds, chewing on peach twigs, and making “tea” from peach leaves."
Resources and Further Reading:
"Cyanide in Apricot & Cherry Pits" ArtOfDrink.com [link]