Blood Thinners Can Come With Dangerous Side Effects. medlineplus.gov. October 3, 2017. Newer class of drugs can interact with multiple medications, researchers report. Blood-thinning drugs can save your life by preventing a heart attack or stroke caused by artery-blocking blood clots. But these are powerful drugs, and a pair of new studies detail side effects people need to understand before taking them. The effectiveness of a class of blood thinners called nonvitamin K oral anticoagulants (NOACs) can be significantly altered through interaction with other drugs, the first study reveals.*NOACs = nonvitamin K oral anticoagulants

In some cases, these drug interactions increase a person’s risk of life-threatening bleeding in locations such as the brain and gastrointestinal tract. In other cases, the NOACs’ effectiveness is reduced, robbing patients of some protection against stroke and heart attack.
"NOACs alone do not pose a significant risk of bleeding, but the concurrent use of NOACs with certain drugs that share the same metabolic pathways may cause increased risk of major bleeding," said study lead researcher Dr. Shang-Hung Chang, an associate professor of cardiology with Chang Gung Memorial Hospital in Taoyuan, Taiwan.
Meanwhile, a second study found that blood thinners can greatly increase a person’s risk of finding blood in their urine.
As a result, patients might unnecessarily wind up in the hospital or emergency room, or undergo an unneeded invasive procedure, said senior researcher Dr. Robert Nam. He is a professor of surgery and head of genitourinary oncology with Sunnybrook Health Sciences Center in Toronto.
"Patients and physicians need to discuss this, to try and prevent patients having to be hospitalized or come to the emergency room in the middle of the night," Nam said.
The first study looked at the bleeding risk associated with NOAC drugs dabigatran (Pradaxa), rivaroxaban (Xarelto) and apixaban (Eliquis).
These drugs are primarily used to prevent risk of stroke in people with atrial fibrillation, an abnormal heart rhythm that can cause blood to pool and clot inside the heart, said Dr. Deepak Bhatt. A spokesman for the American Heart Association, he is also executive director of interventional cardiovascular programs at Brigham and Women’s Hospital’s Heart & Vascular Center in Boston.
NOACs are being used more frequently because they’re easier to use and produce fewer side effects than warfarin, an older anticoagulant that has many food and drug interactions, said Bhatt.
Chang and his colleagues also decided to investigate whether NOACs might have previously unknown interactions with other commonly used medications. The team analyzed health data on 91,330 Taiwanese patients with atrial fibrillation who were prescribed an NOAC.
The investigators found that bleeding risk increased significantly when NOACs were used in combination with amiodarone, fluconazole, rifampin and phenytoin — four drugs that treat widely different conditions.
The researchers also found that other drugs dampened the effectiveness of NOACs, including atorvastatin, digoxin, and erythromycin or clarithromycin.
Bhatt said he’s particularly concerned about the effect of atorvastatin (Lipitor) on NOACs’ effectiveness.
"That’s a very commonly prescribed cholesterol-lowering drug, especially now that it’s generic," Bhatt said. In fact, the researchers found that atorvastatin was the drug most commonly prescribed alongside an NOAC.
"That’s a big deal because that means all those patients on both drugs have a lower risk of bleeding, but on the flip side then would have a higher risk of stroke," Bhatt said.
The second study found that people are much more likely to go to the hospital for blood in their urine if they’re taking blood thinners.
Nam and his colleagues examined medical data on 2.5 million Ontario residents, including nearly 809,000 who had been prescribed a blood thinner.
During an average follow-up period of 7 years, people on blood thinners were six to 10 times more likely to wind up hospitalized or in the ER complaining of blood in their urine compared with others not taking the drugs, Nam said.
Although blood in urine is alarming and requires investigation, Nam said the side effect is not immediately dangerous. Patients should be informed of the potential for blood in the urine, and counseled what to do if it occurs.
"The result of these findings should not affect the decision to treat with these drugs," Nam said. "The benefits outweigh the risks. These are not life-threatening complications. Patients should not stop taking these drugs because of the potential side effect."
In at least one way, this side effect can be welcome, said Dr. Vincent Bufalino, an AHA spokesman and cardiologist in Naperville, Ill.
"Blood thinners can be an advantage because when they cause bleeding in someone on a normal dose, usually there’s a medical issue," Bufalino said. "It actually helps you uncover a problem."
The researchers found that rates of bladder cancer detection were twice as high in people taking blood thinners compared to the general population. The drugs made the tumor produce more blood, Nam said.
"Because it bled, they went in, looked, and found the bladder cancer and were able to get it earlier than would have happened waiting for a patient to develop symptoms on their own," Bufalino said. "In many of these circumstances, the cancers would have been missed until they were more advanced because the patient wouldn’t have known there was a problem."
The two studies were published in the Oct. 3 issue of the Journal of the American Medical Association.
SOURCES: Shang-Hung Chang, M.D., Ph.D., associate professor of cardiology, Chang Gung Memorial Hospital, Taoyuan, Taiwan; Robert Nam, M.D., professor of surgery and head of genitourinary oncology, Sunnybrook Health Sciences Center, Toronto; Deepak Bhatt, M.D., executive director of interventional cardiovascular programs, Brigham and Women’s Hospital Heart & Vascular Center, Boston; Vincent Bufalino, M.D., cardiologist, Naperville, Ill.; Oct. 3, 2017, Journal of the American Medical Association
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