Blood Thinners Side Effects

Blood Thinners Side Effects – Overview

Heart attacks and strokes are well-known life-threatening medical conditions. Every 40 seconds, one heart attack and one stroke happen to people in the U.S. Moreover, 200,000 people have another heart attack every year, and about 185,000 strokes occur in those who previously had one. (1-2)

Blood clots can cause stroke, heart attack, and other diseases and cause 100,000 deaths yearly. Fortunately, medications can save many lives. Blood thinners prevent blood clots from forming to help prevent these life-threatening conditions. (3)

Still, some patients are concerned about bleeding, sudden headache, dizziness, or other blood thinner side effects and can become insecure and afraid to take these medications. If you are one of these individuals, this article will explain the possible side effects.

Keep reading to learn about the serious side effects, when to call a doctor, and how to prevent complications while maintaining the blood thinners’ effectiveness.

Blood Thinners Can Save Lives

Let’s see the role of blood thinners to help understand the side effects better. Blood thinners or anticoagulant medications prevent blood clots from forming or existing clots from growing.
In a healthy individual, blood clots have a protective role in stopping bleeding from an injury.

Blood Thinners Side Effects

Blood Thinners Side Effects – Blood Thinners Can Save Lives

But, in some cases, clots are formed more than needed or in a part of the body where they shouldn’t be. Blood clots in the lungs can get stuck in an artery, damage the tissue, and affect the body’s oxygen supply (pulmonary embolism). In the brain, blood clots might block blood vessels and cause decreased oxygen supply to the organ (ischemic stroke). In the same way, a blood clot that comes into the heart blood vessels leads to blockage of the blood supply producing a heart attack. (4-5)

Those and some other conditions (Deep vein thrombosis, Pulmonary embolism) caused by blood clots can be dangerous and life-threatening. Blood thinners are prescribed to prevent stroke, heart attack, or pulmonary embolism, saving thousands of lives.

There are two main groups based on the mechanism they act:

  • Anticoagulants prevent blood clots from growing bigger, or they prolong blood clotting time;
  • Antiplatelets don’t allow platelets to stick, which prevents clots from forming. (4)

In this way, clot-preventive medications protect the body from harmful clots that can cause severe damage or death.

Blood Thinners’ Side Effects To Be Aware Of

It’s nothing new that blood thinners, like other drugs, can cause side effects. Still, understanding side effects, their causes, and the factors that increase their risks is a good way to get awareness, prevent complications, and ensure safe and effective therapy.


Blood thinners reduce the body’s ability to form clots. Consequently, it can result in prolonged or heavy bleeding, one of the most common side effects.
In many cases, bleeding can be easily noticed, such as bleeding from gums, nose, or minor cuts that take longer than usual to stop. Easy bruising could also happen while on blood thinners. (6)

Heavy Bleeding Period In Women

Heavy menstrual bleeding could happen in women who take anticoagulant medication. A recent study published in the medical journal Blood suggests that two out of three women experience abnormally heavy periods while taking blood-thinning medication to treat venous thromboembolism (blood clots forming in a vein). Abnormal uterine bleeding usually occurs in 10-30% of women in their fertile ages. (7)

Heavy period bleeding can reduce the quality of life, make women weak or tired, negatively reflect on work capabilities, and limit social activities. You should talk to your doctor if this happens. (8)

Bleeding Inside The Body

In rare cases, the bleeding can be severe and dangerous. Intensive bleeding might occur inside the body and become visible with vomiting, urine, or stool. Blood changes the color of urine to red or pink or the color of stool to bright red, dark, or black.

Bleeding might occur in a joint or the head after a sudden hit. If you have a sudden headache, hit your head, fall, or notice color changes in urine or stools, you need immediate care, so call your doctor or 911 right away. (9)

Bleeding as a side effect is a challenge for scientists to overcome. However, the results from a study performed by researchers from the U.S., Switzerland, and Germany suggest that we can expect bleeding-free anti-coagulation therapy in the future (10). Until sufficient data becomes available, doctors have several effective blood thinners for treatment.

The aim of the treatment, as Dr. Gregory Piazza, associate professor of medicine at Harvard Medical School, explains, is to find the right balance between helpful and harmful clotting (11). With the patient’s proactive role in reporting any occurred event, the doctor can treat the condition successfully and safely.

Feeling Tired, Dizzy, Or Weak

Tiredness, weakness, or dizziness limits people from performing daily activities. Yet, while on blood thinners, weakness and dizziness might signal the presence of low blood counts.

Some patients using certain anticoagulant medications have reported these side effects with various frequencies and severity. Those who are on blood thinners and feel tired need to call a doctor immediately for medical help. (12-14)

Call a doctor immediately if you use an anticoagulant and feel dizzy or weak. (15-18) Sudden weakness in the face, arm, or leg, especially on one side of the body, or severe headache, are signs of a stroke. (19) If you feel dizzy, be careful when you stand up; in this case, you should lie down until you feel better and slowly stand up. (20-21)

Other Drug-Related Side Effects

Other side effects that might occur while on blood thinner treatment are related to certain medications: (22)

  • Skin necrosis – This can rarely happen while on warfarin treatment. Warfarin can paradoxically cause forming clots in the arm, leg, or other parts of the skin. These blood clots block the blood flow, and the affected skin area gets damaged;
  • Blue or purple toe – This painful condition might happen in the first two months of warfarin treatment, appearing as toes and feet that change in color;
  • Heparin-induced thrombocytopenia (low platelet counts)– It can present as a slight decrease of platelets, which is more common and less dangerous. In contrast, the severe type causes platelet activation, which triggers a potentially dangerous clotting chain reaction. However, the severe type occurs in 1% of those who receive heparin;
  • Osteoporosis (decreased bone mineral density)– Long-term heparin use (less likely for low-molecular-weight heparin) decreases new bone cells forming and increases the breakdown rate of old bone cells; this can cause osteoporosis;
  • Gastrointestinal bleeding – anticoagulants can sometimes cause indigestion and/or bleeding in the gastrointestinal tract;
  • Aspirin-induced asthma – up to 20% of people with asthma are sensitive to aspirin and other non-steroidal anti-inflammatory drugs; if taken, aspirin can trigger the symptoms of rhinitis, sinusitis, and asthma. (23)

What To Do In Case Of An Adverse Event?

You can find the list of the drug’s side effects in the Medication Guide. Once you get your prescription and Medication Guide, read it carefully, familiarize yourself with the possible side effects, and consult your doctor if you have any questions.

Contact your doctor (or go to the nearest hospital) in case of the following manifestations:

  • Prolonged bleeding from minor injuries;
  • Color changes or blood in urine or stools;
  • Fall or bump on your head; or
  • Feelings of tiredness, weakness, or dizziness.

You or your doctor can report side effects to the Food and Drug Administration; this will help update the safety profile of your medication.

Follow The Recommendations

Keep in mind your doctor’s recommendations on how to use your medication and when to attend to follow-ups. Once you know the possible side effects and risks, you can take preventive measures: (24)

  • Avoid NSAIDs painkillers (ibuprofen or naproxen), which increase the risk of bleeding when administered together with blood thinners;
  • Consume vitamin K-containing food with special care (asparagus, broccoli, cauliflower, parsley, spinach, and others) if you take warfarin because it alters the medication mechanism;
  • Avoid alcohol because it prolongs blood clotting.

When there is a risk of cutting, put on gloves and carefully use a knife, scissors, or other sharp tools at home or outdoors. Stay physically active, yet, choose activities with less risk of falling, such as walking or swimming. (24)


Despite your disease’s severity and side effects risk, you can actively participate in your treatment by reporting your concerns to your doctor. Blood thinners can prevent another heart attack or stroke and save your life. Your medication can do a lot; you can make the treatment safer with your engagement.

It is essential to take your medicine regularly as your doctor prescribes it. Be aware of your condition and be alert to any changes that occur. Sudden headaches, blood in urine or stool, heavy bleeding periods, tiredness, or dizziness are signals to call a doctor and ask for immediate medical care.

Acting when an adverse event occurs and avoiding injury and bleeding will help you to achieve optimal treatment and put your life back on track again.

See Also

Is Tylenol A Blood Thinner?

Do Blood Thinners Make You Tired?

Is Ibuprofen a Blood Thinner?

Do Blood Thinners Decrease Blood Pressure?

Do Blood Thinners Affect Oxygen Levels?

What is Creatinine Level in Blood Tests?

Current Version
December 6, 2022
Written By
Biljana Srbinovska, MPharm
December 6, 2022
Edited By
Franco Cuevas, MD
December 6, 2022
Medically Reviewed By
Tyler Spink, PharmD

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23. K. Suresh Babu, MD et al., Aspirin and Asthma, Chest 2000;118(5)

24. Blood Thinner Pills: Your Guide to Using Them Safely, Agency for Healthcare Research and Quality, November 2018

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