Many seniors pay much more for prescription drugs than Medicare Part D’s $4,700 catastrophic coverage threshold might suggest. 

A new study by the Kaiser Family Foundation finds that many seniors who take expensive drugs for arthritis, hepatitis C, multiple sclerosis and cancer will spend between $4,000 and $12,000 in 2016 just to take one drug. 

Why? Well, the drugs are extremely expensive. So expensive that Medicare beneficiaries who depend on them are footing five-figure bills even though they are only picking up 5 percent of the tab. 

Those on such expensive drugs quickly hit the threshold in the first months of the year, when they are picking up 25 percent to 33 percent of the cost (and sometimes a deductible). They then hit a gap period, when they’re responsible for 45 percent of the cost of brand-name drugs and 65 percent for generics. Afterwards, they hit the catastrophic phase. 

The most expensive median cost comes from Revlimid, a drug used to treat some blood cancers. A senior on Revlimid spends a median of $11,538 a year, the great majority of which comes after catastrophic coverage is in effect. 

Most seniors reach the catastrophic threshold from multiple prescriptions. For instance, the typical Revlimid user reaches the threshold after spending about $3,000 on Revlimid and the rest on other medications.  

In a distant second place is Gleevec, a drug mainly used to treat leukemia. Medicare beneficiaries who take it can expect to spend $8,503 a year just for that medication. Zytiga, another cancer treatment, comes in third at $7,227. 

Those taking drugs for Multiple Sclerosis or Hepatitis C cost spend a median of between $6,000 and $7,000 a year for their prescriptions. Typically between half to 60 percent of that cost is incurred after the catastrophic threshold is reached. 

Arthritis is cheaper, but pricey nevertheless. The median cost for a senior on Orencia, Humira or Enbrel is $4,413, $4,864 and $4,872 a year, respectively. But unlike those taking medication for the more serious, life-threatening diseases, arthritis patients typically spend most of their money before hitting the catastrophic threshold.