According to the National Capital Poison Center, begonias are not toxic to humans. However, begonias, especially their tubers, are highly poisonous to household pets, such as dogs and cats.
Begonias, with over 1,000 species and 10,000 hybrids, contain insoluble oxalates that can kill dogs and cats. Manifest symptoms that indicate a pet might have begonia poisoning include drooling, vomiting, problems swallowing, burning and visible irritation of the mouth, lips and tongue. Any pet that has chewed or ingested begonias should be treated immediately by a veterinarian or according to the protocol advised by experts at an ASPCA Animal Poison Control Center.