At 1/4 to 2 inches in diameter, crabapples look like miniature versions of regular apples. However, a crabapple has a sour, pithy taste. Despite their unpleasant sharpness, crabapples generally aren't dangerous. Crabapples might give you a sore stomach if you eat a few, but they won't cause poisoning. Like regular apples, their seeds do contain toxins. However, you would have to eat them in huge doses to experience any serious problems.
Crabapples usually aren't grown for their fruit. The little crabapples are so sharp and bitter that most people just spit them right out again. You can use some slightly mellower larger crabapple varieties for jam making. Flowering crabapple trees are attractive in gardens. Like their close cousin, the apple, crabapple trees often bloom in pretty white or purple flowers. This makes them useful as an ornamental species.
Crabapples are essentially the same species as apple trees. The seeds of both of these trees contain a form of cyanide called cyanogenic glycosides. Cyanide is a toxic, potentially fatal poison. However, the average American eats around 16.9 pounds of fresh apples every year and reports no toxic effects. Despite the presence of cyanide in the seeds, most people don't eat the core. Even when apple seeds are ingested, they usually pass through the gut without being broken down. You would have to eat lots of crabapple seeds and grind or chew them up for the cyanide to take effect.
Crabapples may present more danger for pets, such as dogs. Animals that consume a few crabapples may show signs of discomfort, according to experts at the ASPCA. In serious cases, if a pet eats lots of crabapples, including stems, leaves and seeds, they could show signs of cyanide poisoning. Dogs rarely eat enough plant matter for this to cause a real problem. Horses, sheep, cows and other herbivorous animals may eat crabapples in large enough quantities to trigger some toxic effects.
If you're worried that crabapple fruit is dangerous, remove the core, stem, leaves and any seeds from the fruit before use. These are the only parts with the toxic cyanogenic glycosides. The tart flesh of crabapples doesn't contain the poisonous substance. You can use the white flesh alone in your jams, jellies and bitter tarts without exposing yourself to danger. Of course, as with any fruit, avoid rotten or moldy fruit which can cause food poisoning.