More than a quarter of U.S. kids and teens are taking a medication on a chronic basis, according to Medco Health Solutions Inc., the biggest U.S. pharmacy-benefit manager with around 65 million members. Nearly 7% are on two or more such drugs, based on the company's database figures for 2009. Doctors and parents warn that prescribing medications to children can be problematic. There is limited research available about many drugs' effects in kids. And health-care providers and families need to be vigilant to assess the medicines' impact, both intended and not.