Gage Martindale, who is 8 years old, has been taking a blood-pressure drug since he was a toddler. “I want to be healthy, and I don’t want things in my heart to go wrong,” he says.

And, of course, his mom is always there to check Gage’s blood pressure regularly with a home monitor, and to make sure the second-grader doesn’t skip a dose of his once-a-day enalapril.

These days, the medicine cabinet is truly a family affair. 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. Although the effects of some medications, like cholesterol-lowering statins, have been extensively researched in adults, the consequences of using such drugs for the bulk of a patient’s lifespan are little understood.

Many medications kids take on a regular basis are well known, including treatments for asthma and attention-deficit hyperactivity disorder.

But children and teens are also taking a wide variety of other medications once considered only to be for adults, from statins to diabetes pills and sleep drugs, according to figures provided to The Wall Street Journal by IMS Health, a research firm. Prescriptions for antihypertensives in people age 19 and younger could hit 5.5 million this year if the trend though September continues, according to IMS. That would be up 17% from 2007, the earliest year available.

Researchers attribute the wide usage in part to doctors and parents becoming more aware of drugs as an option for kids. Unhealthy diets and lack of exercise among children, which lead to too much weight gain and obesity, also fuel the use of some treatments, such as those for hypertension. And some conditions are likely caught and treated earlier as screening and diagnosis efforts improve.