(CNN) New guidelines tackle a question many older adults on blood pressure medication face: How low should you go? But finding the right blood pressure might not be so straightforward.

The latest guidelines , released yesterday by two physicians' groups, have already drawn criticism for how high they set targets for adults 60 and older, even in light of recent data that some patients, especially those at higher risk for stroke and heart attack, may benefit from lower blood pressures.

According to the new guidelines, those with a history of stroke or heart attack should aim for a target systolic blood pressure below 140, the lower limit for what is considered high blood pressure. Otherwise, the majority without these risk factors should be treated to achieve a number below 150. If their numbers fall between 140 and 150, still considered high by many doctors, it may not be worth treating them at all.

"It's about weighing the harms and the benefits," said Dr. Devan Kansagara, an internist at the VA Portland Health Care System, who co-led an analysis of existing studies , also released yesterday, that provided the basis for these recommendations.

On one hand, aiming below 140 or even below 130 may reduce the chance of stroke, heart attack and other cardiac problems in some patients, said Kansagara. But it does not seem to change the overall death rate or quality of life.

Increasing medication may also raise certain risks, such as fainting and abnormal kidney function. However, the risks of falls and dementia do not significantly increase, according to the analysis. None of the studies included in the analysis showed any drugs to be more effective, or more risky, than others.

Kansagara warned that more research is needed to understand and personalize treatment for patients with high blood pressure.

"This is really a moving target," said Dr. Nitin Damle, president of the American College of Physicians, which developed the new guidelines with the American Academy of Family Physicians.

What's in a number?

High blood pressure, or hypertension, affects about a third of adults in the United States, according to the Centers for Disease Control and Prevention, and only about half of them have it under control. Many do not know they have it at all.

Adults 60 and older have twice the rate of hypertension as the general population, with two-thirds affected, according to the National Center for Health Statistics (PDF).

Blood pressure may increase due to stress, diet, medical conditions and even an anxiety-inducing visit to the doctor. Medication is one of many ways, including exercise and lowering salt intake, that people control chronic hypertension.

When it comes to targeting a specific blood pressure, the research is skewed, said Kansagara, who is a member of ACP but did not vote on the new guidelines. The largest share of the data comes from only two clinical trials: the Systolic Blood Pressure Intervention Trial (SPRINT), and Action to Control Cardiovascular Risk in Diabetes (ACCORD). Both trials, which aimed to push hypertensive patients below 120, showed vastly different results.

The SPRINT researchers concluded in late 2015 that lower was indeed better. When the researchers noticed that the patients with lower blood pressures had far better outcomes, they stopped the trial early, a controversial practice that could inflate how effective their methods appeared, said Kansagara.

The SPRINT study rattled many doctors who were now faced with the prospect of treating hypertension more aggressively, and with more medication on average, than prior guidelines. It was also a key impetus for Kansagara's review.

"To go down to 120 overnight is a fairly large change in practice," said Kansagara. "We wanted to look at this impactful trial in the context of all of the other evidence."

ACCORD, a smaller trial that preceded SPRINT, did not show the same benefit. Though similar in some ways, the trials had different types of patients. For example, ACCORD included only diabetic patients, while SPRINT excluded them. SPRINT targeted certain patients at higher cardiovascular risk, who were also older on average.

"Apples and oranges," said Damle, an internist in Wakefield, Rhode Island. "They are two separate trials that are not totally inconsistent."

Damle said that there was only "weak evidence" that the physicians' groups should set the bar lower than they did. "Be aware that if you're going lower, that you're risking other side effects," he said.

AAFP president Dr. John Meigs added that there is flexibility in the guidelines, stressing the importance of having a relationship with a primary care doctor.

"It really does help knowing a patient over time," said Meigs, a family physician. "I've got patients I've been treating for 35 years."

Some of Meigs' patients can add a new blood pressure medication without any issues or side effects, but for others, more medication might be a cost or medical burden.

Another study from Johns Hopkins University, published last week, suggested that CT scans may be an additional tool to help personalize targets for patients with moderately high blood pressures.

"Ultimately, it's the patient's choice," Meigs said.

Tightening the cuff

The American Heart Association has remained firm on its recommendations to take action for many older adults once their blood pressure hits 140.

"If I have a blood pressure over 140, I want my physicians to treat me before I get a stroke, not after," said association President Steven Houser, who is not a physician.

"The data is absolutely clear," he added.

Houser is "personally convinced that there's a benefit" to targeting lower numbers.

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Houser said that AHA's own recommendations are currently under review, and that the results of that review may be ready this summer. He said that they are "looking at every study" that has come out since their last recommendations, including the SPRINT study.

"We still have something like 80 million people (in the US) that are still hypertensive," said Houser, adding that each one of those people may have different health needs and goals, a point that AAFP's Meigs agrees with.

"The guidelines are straightforward," said Meigs. "But people are complicated."