The active ingredient cantharidin in the aphrodisiac known as Spanish fly has a number of potentially dangerous side effects, reports PubMed. Cantharidin is a potent inflammatory substance, and ingestion of the substance in large quantities can cause impairment of consciousness, vomiting of blood, impaired kidney function, coma and even death in extreme cases.
Cantharidin is produced by a family of beetles that includes the insect known as a Spanish fly, writes PubMed Central. The chemical acts as an irritant and is stored in the lymph nodes and genitalia once ingested, where it functions as an aphrodisiac by stimulating erection in males. However, this erection may also develop into a priapism, a dangerous and continuous state of erection where blood becomes trapped in the penis. Cantharidin has also been used as a chemical method to induce abortions in women due to its irritant properties.
Most doses of substances sold purported to be Spanish fly contain negligible amounts of cantharidin, writes PubMed. This likely renders these concoctions harmless unless other dangerous chemicals are present as well as ineffective as aphrodisiacs. However, some illicit preparations of Spanish fly may contain enough cantharidin to pose a danger to the health of people who ingest them.