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.