Of American seniors on medications, 51% take at least 5 different prescription drugs regularly, and 25% take between 10 and 19 pills each day, according to a study by a prescription mail-order firm.

The study by Medco Health Solutions Inc., Franklin Lakes, N.J., found 57% said they forget to take their medications. Among those using 5 or more medications, 63% said they forget doses, compared to 51% of those who take fewer medicines.

The survey also found 34% of seniors using 5 or more prescription drugs do not feel knowledgeable enough about the drugs they are prescribed or their potential side effects, and 35% are not sure they can name all the medications they use.

The ability to afford their drugs was the top concern among 40% of seniors taking 5 or more prescriptions daily, followed by their worry over drug side effects (23%) and concern over interactions with other drugs (17%).

Among those enrolled in a Medicare Part D plan, 37% were most concerned about not being able to afford their medications, and 49% wanted to know how to delay or avoid falling into the donut hole or coverage gap-the phase in which the beneficiary is responsible for the full cost of their drugs.

According to the survey, 60% of Medicare beneficiaries have taken some steps to delay reaching the donut hole, such as using generic medications (76%) and asking for free samples from their doctors (39%). But just 27% said they use mail order to avoid the donut hole, despite the fact that surveyed seniors estimated that using mail order saved them about $540 a year on average, Medco says.

The study was conducted by Kelton Research among more than 1,000 people ages 65 and older who use medications.