When people talk about expensive drugs, they usually are referring to drugs like Lipitor for high cholesterol ($1,500 a year), Zyprexa for schizophrenia ($7,000 a year) or Avastin for cancer ($50,000 a year). But none of these medicines come close to making Forbes magazine’s exclusive survey of the most expensive medicines on the planet.  The nine drugs on the Forbes list all cost more than $200,000 a year for the average patient who takes them – mostly prescribed to treat rare genetic diseases, afflicting fewer than 10,000 patients, for which there are few if any other treatments.

So drug companies can charge pretty much whatever they want. Selling drugs for rare diseases has become immensely profitable. There are so few patients that companies don’t have to invest as heavily in marketing. And the medicines usually get paid for by insurers or government health care plans.  

Alexion Pharmaceutical’s Soliris, at $409,500 a year, is the world’s single most expensive drug. This drug treats a rare disorder in which the immune system destroys red blood cells at night. The disorder, paroxysymal nocturnal hemoglobinuria, affects about 8,000 patients in North America.

Last year Soliris sales were $295 million. Since Alexion started selling Soliris two years ago, its stock price is up 130%.

Read the rest of the Forbes article to see the full list of the world’s most expensive drugs.