PHILADELPHIA (AP) -- Patients are often harmed by inadequate care and outright medical mistakes in the days after they are sent home from the hospital, according to new research.

The study, conducted at one large hospital, found that nearly one in five patients had "adverse events" after they go home - new or worsening symptoms resulting from the treatment they received, not from their underlying disease. Most problems could have been prevented or eased with better care.

The researchers said the problems often occur because hospitals fail to communicate effectively with patients and their primary care physicians after discharge, and neglect to follow up to identify symptoms and complications before they become more serious.

Many studies have looked at patient safety inside hospitals, including a review by the Institute of Medicine that blamed medical mistakes for the deaths of 44,000 to 98,000 hospitalized Americans each year. The latest report, in Tuesday's Annals of Internal Medicine, is the first to assess how often discharged patients become sick as a result of their treatment.

Researchers at the University of Ottawa and Harvard Medical School contacted 400 patients who were hospitalized at an unidentified urban teaching hospital.

They found 76 patients had adverse events after they were sent home. Of those, 23 were deemed preventable and 24 would have been less severe with better care.

Two-thirds of the problems resulted from drug side effects. In one case, an asthmatic patient who had a heart attack was prescribed a beta blocker, a drug that slows the heart rate but can cause asthma attacks. The patient developed wheezing and a cough.

In another case, a patient with an inflamed pancreas was sent home after his X-ray was misread. He was readmitted four days later with worsening symptoms.

Study co-author Dr. David Bates said the results demonstrate a need for better follow up.

"The current reimbursement structure does not reward providers for giving post-discharge care," but "hospitals ought to support having someone get in touch with (discharged patients)," said Bates, of Harvard's Brigham and Women's Hospital.

The study's results are not surprising because patients are discharged from the hospital more quickly than in the past - and in worse shape, said Dr. Kenneth Kizer, president of the National Quality Forum, which is working to develop better ways of measuring medical care. "They are still vulnerable, their needs still have to be looked after and they need to be tended to."

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