Two new studies out today found that patients may not be prescribed the best possible dosages of common heart medications.

The first study, published in the Journal of the American College of Cardiology, examined newer blood thinners such as Eliquis and Zarelto and found that about one in six patients isn't getting the right dose, which could increase their risk of serious complications, such as bleeding. And a second study, published in the journal Hypertension, suggests that many patients taking medication for high blood pressure could be taking much lower dosages of multiple drugs instead, with fewer side effects.

“If you are taking any of these prescriptions, you should talk to your doctor to make sure you are getting the best dose for you,” advises Marvin M. Lipman, M.D., Consumer Reports’ chief medical adviser.

Here’s what you need to know about the new findings.

Blood Thinners: Getting the Right Dose

Millions of Americans take a blood-thinners such as apixaban (Eliquis), rivaroxaban (Xarelto), or dabigatran (Pradaxa) for atrial fibrillation, a heart condition that causes an irregular and rapid heart rate and which can quadruple the risk of stroke. These newer drugs have all been shown to be as effective as warfarin (Coumadin), an older blood-thinning medication, with fewer side effects.

But the new study in the Journal of the American College of Cardiology, which was led by researchers at the Mayo Clinic in Minnesota and looked at almost 15,000 patients on these newer drugs, found that 16 percent of them received the wrong dose—potentially undermining the medications’ effectiveness and increasing the possibility of life-threatening complications.

The medications are currently available in two dosages: a standard dosage and a lower dosage for patients with kidney problems (about 10 percent of patients with atrial fibrillation also have severe kidney disease). Yet 43 percent of patients with kidney problems received the standard dose, increasing their risk of major bleeding, while about 13 percent of patients without kidney disease received the less effective lower dose.

“Neither group saw any benefit: The overdosed group didn’t reduce their risk of stroke any further, while the underdosed group didn’t have any reduction in bleeding risks,” says Jonathan Halperin, M.D., professor of cardiology at the Icahn School of Medicine at Mount Sinai in New York.

So why are doctors prescribing incorrect doses? It’s a combination of both extra caution and, at times, carelessness, study authors say.

“If a physician has an 85-year-old patient come in with atrial fibrillation, they are often reluctant to treat her with a full dose,” says study author Peter Noseworthy, M.D., adding that a full dose of apixaban (Eliquis) might be best in those cases. “On the opposite end of the spectrum, a younger, seemingly healthy patient may come in and the physician will just write a prescription for a higher dose without even thinking about their kidney function.”

Blood Thinners: What to Ask Your Doctor

If you or someone you care for takes one of these medications, ask your physician these questions.

What Is My Current Kidney Function?
Your doctor should check your kidney function once a year, and even more frequently if you already have kidney disease, says Noseworthy. That way, your dosage can be scaled up or down accordingly.

Why Are You Giving Me This Dose?
Knowing whether you have been prescribed the higher dose or the lower dose of your blood thinner is important. (If you take apixaban, the low dose is 2.5 mg and the high dose is 5 mg. If you take dabigatran, a high dose is 150 mg, and a low dose is 75 mg. If you take rivaroxaban, the high dose is 20 mg and the low dose is 15 mg.)

If you're on the high dose, make sure that your doctor has ruled out any kidney disease. But if you’re on the low dose, be sure to ask why—your doctor may be overly cautious. “Sometimes it’s as simple as a doctor looking at a patient and saying, ‘I don’t like that bruise, so let’s be cautious and prescribe a lower dose,” says Halperin.

Can Any Other Drugs I’m Taking Have an Impact on the Dose?
Antidepressants such as paroxetine (Paxil) and sertraline (Zoloft), antibiotics such as rifampin (Rifadin) or clarithromycin (Biaxin) or the anticonvulsant carbamazepine (Carbatrol) may all interact with blood thinners, so make sure your doctor knows everything you're taking. He or she may need to adjust your dose, or monitor you closely for side effects like bleeding.

Hypertension Meds: When Smaller Doses Are Better

Many people find blood pressure medications hard to tolerate, since side effects can include dry cough, stomach upset, dizziness, headache, and fatigue.

But the new study in the journal Hypertension suggests that smaller doses of multiple drugs work better and create fewer side effects than a large dose of a single drug.

When researchers analyzed results from over 42 studies of more than 20,000 people with high blood pressure, they found that combining two medications, each at a quarter dose, was just as effective as taking one blood pressure medication at a standard dose—and taking four medications each at a quarter dose worked twice as well. 

“This study confirms what many of us already anecdotally knew in our own practices: that lower doses of multiple medications are often the way to go, because it seems to reduce side effects, especially in women,” says Nieca Goldberg, M.D., medical director of NYU Langone’s Joan H. Tisch Center for Women’s Health.  

The reason the approach may be so effective, theorizes Goldberg, is because it works on all the different aspects of hypertension: For example, an ACE inhibitor like lisinopril (Prinivil, Zestril) relaxes blood vessels, while a diuretic such as HCTZ (Esidrix, Hydrodiuril, Microzide) gets rid of the extra salt and water in your body that can elevate your blood pressure.

Hypertension Meds: What to Ask Your Doctor

While 88 percent of patients who know they have high blood pressure are being treated with medications, only about one in three are able to gain control of their hypertension, according to the review.

If you can’t seem to get your blood pressure under control, talk to your doctor about possibly combining at least two medications.

“If people are doing fine with a single drug, then there is no need to change,” says study author Anthony Rodgers, M.D., Ph.D., professor of medicine at the George Institute for Global Health in Newtown, Australia. “But research shows that most people require more blood pressure lowering than what can be achieved with a single drug.”

The one exception, warns Goldberg, is people with very high blood pressure—of over 160/90. “Their blood pressure is so elevated that they may need higher doses of medication, and the review didn’t look at cases like that,” she says.

It’s also important to realize that drugs themselves aren’t a cure-all. “It’s not enough to just take your medications reliably—you need to lose weight, exercise, and reduce salt as well,” says Rodgers.