Hawthorn berries are not directly poisonous, but there are certain circumstances in which they can have adverse effects. The seeds in Hawthorn berries contain a compound called amygdalin, which is cyanide bonded with sugar. This changes to hydrogen cyanide in the small intestine, and the result can be deadly.
It is best to spit out Hawthorn seeds when consuming the whole berry. A few seeds are not necessarily going to cause a problem for an adult, but a child is likely to be adversely affected. The berries themselves are not poisonous, but many people report an unpleasant aftertaste from eating the fruit. Large doses of Hawthorn have been reported to be toxic.
Hawthorn has a variety of medical uses and is used in supplement form to treat various maladies. Specifically it is used to treat heart failure, especially mild forms of congestive heart failure. It is less effective for severe forms of heart failure. The berries are used in teas to lower blood pressure as well.
Hawthorn berries are also widely used in cooking jellies because they are naturally high in pectin. However, Hawthorn berries have little taste of their own and are usually mixed with other fruits in items such as pies or wine.