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
The Committee of Experts on Flavourings of the Council of Europe 1 mg/kg
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."
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.
Resources and Further Reading:
"Cyanide in Apricot & Cherry Pits" ArtOfDrink.com [link]