Up to 40 percent of U.S. seniors do not take all their prescription drugs, either because of the cost or because they do not think they need them, according to a survey published on Tuesday.

Twenty-six percent said they did not fill a prescription or took smaller or fewer doses because they could not afford it, and a quarter said they skipped doses or stopped taking a drug because it made them feel worse or was not helping.

The survey of nearly 18,000 Medicare beneficiaries 65 and older found that drug coverage made a big difference, with 37 percent who lacked such coverage saying they had trouble affording medicines compared to 22 percent who were covered.

The survey, conducted in 12 states in 2003 by the Kaiser Family Foundation, the Commonwealth Fund and Tufts-New England Medical Center, found that 89 percent of those questioned had taken prescription drugs in the past year.

About 46 percent said they took five or more drugs, 54 percent had more than one doctor prescribing drugs for them and 35 percent used more than one pharmacy.

“With two out of five seniors not taking medicines as prescribed, there is a real opportunity to improve patient care both by urging doctors and patients to talk more about these issues and by developing systems to monitor quality and safety,” said Commonwealth Fund President Karen Davis.

“The substantial variations in drug coverage across states documented in this survey suggest that targeted outreach efforts have the best chance of reaching seniors,” added Kaiser Family Foundation President Drew Altman.

Five percent of those surveyed said that had bought prescription drugs from pharmacies in Canada or Mexico. Eleven percent of seniors who did not have drug coverage got their medications abroad.