For decades, Don Anderson of Seattle has been taking the same drug to help control the temporary bouts of immobility and muscle weakness caused by a rare and frightening genetic illness called periodic paralysis. The drug Anderson has been taking all these years was originally approved in 1958 and used primarily to treat the eye disease glaucoma under the brand name Daranide. The price has been on a roller coaster in recent years - zooming from a list price of $50 for a bottle of 100 pills in the early 2000s up to $13,650 in 2015, then plummeting back down to free, before skyrocketing back up to $15,001 after a new company, Strongbridge Biopharma, acquired the drug and relaunched it this spring. The zigzagging trajectory of the price of Daranide, now known as Keveyis, shows just how much freedom drug companies have in pricing therapies - and what a big business opportunity selling extremely-rare-disease drugs has become. In 2016, after The Washington Post asked questions about the high price of the drug, Sun Pharmaceutical said it would give the drug away free. Late last year, Sun agreed to sell Keveyis to a biotech company, Strongbridge Biopharma. In April, Strongbridge relaunched the drug. In August, it jacked the list price ... to $15,001 for a bottle of 100 pills. In a PowerPoint presentation for investors, Strongbridge Biopharma estimated that the annual price of treatment for the drug, Keveyis, would range from $109,500 to $219,000.