Blood Thinners Side Effects

Blood Thinners Side Effects 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. and can happen again. Every year, 200,000 people have another heart attack, and about 185,000 strokes occur in those who already have one. (1-2)

Blood clots can be the cause of stroke, heart attack, and other diseases and are the reason for 100,000 deaths each year. Luckily, medications can save many lives. Blood thinners prevent forming of blood clots, so don’t allow harmful clots to cause life-threatening conditions. (3)

Still, some patients are concerned about bleeding, sudden headache, dizziness, or other blood thinners’ side effects and become insecure and afraid to take the therapy. If you are one of those, this article will explain all the possible side effects.

Keep reading to learn about the serious ones, when to call a doctor, how to prevent complications, and maintain the blood clots’ treatment effectiveness.

Blood Thinners Can Save Lives

Let’s see the role of blood thinners, good and bad clots, to understand the side effects better. Blood thinners or anti-clotting medications prevent the forming of blood clots or prevent made clots from growing bigger. 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 bleeding (ischemic stroke). The blood clot that comes into the heart leads to blockage of the blood flow (heart attack). (4-5)

Those and some other conditions (Deep vein thrombosis, Pulmonary embolism) caused by blood clots can be dangerous and life-threatening. Anti-clotting medications are prescribed to prevent stroke, heart attack, or pulmonary embolism and can save 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 and help clots to form. (4)

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

Blood Thinners’ Side Effects to Be Aware of

Nothing new that blood thinners, like other drugs, can cause side effects; still, some patients feel insecure. Understanding side effects, their reasons, and the factors that increase their risks is the only way to overcome the fear, prevent complications, and ensure a safe and effective therapy.


Blood thinners regulate the bad blood clots but affect the function of the good ones; this results in prolonged or heavy bleeding. Bleeding is the most common side effect. 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 blood clots treatment. An observational study published recently 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 only 10-30% of women of reproductive age. (7)

Heavy period bleeding can reduce the quality of life, make weak or tired, negatively reflect on work capabilities, and limit women to enjoy social activities. If this happens to you, talk to your doctor. (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 urine color to red or pink, or the stool is bloody, dark, or black.

Bleeding might occur in a joint or in the head after a sudden hit. If you have a sudden headache, hit your head, fall, or notice color changes in urine or stool, you need immediate care, so call your doctor or 911 immediately. (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 successfully and safely treats the disease.

Feeling Tired, Dizzy, or Weak

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

Some patients using certain anti-clot medications have reported these side effects with various frequencies and severity. Those who are at heparin, warfarin, or prasugrel treatment and are tired need to call a doctor immediately to get medical help. (12-14)

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

Other Drug-related Side Effects

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

  • Skin necrosis – This can rarely happen while on warfarin treatment. Warfarin can cause forming clots in the arm, leg, or other parts of the body. These blood clots block the blood flow, and the affected skin area dies;
  • Blue or purple toe – This painful condition might happen in the first two months of warfarin treatment when toes and feet change color;
  • Heparin-induced thrombocytopenia – Type I is a slight decrease of platelets, more common and not dangerous. Type II is platelet activation that triggers a chain reaction of clotting. Type II occurs in 1% of those who receive heparin and is extremely dangerous; Osteoporosis – Long-term heparin use (less likely for low-molecular heparin) decreases new bone cells forming and increases the breakdown rate of old bone cells;
  • Gastrointestinal bleeding – direct anticoagulants can sometimes cause indigestion 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 triggers 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, please read it carefully, familiarize yourself with the possible side effects and consult your doctor if needed.

If you have prolonged bleeding, see color changes or blood in urine or stool, fall or bump on your head, or feel tired, weak, or dizzy, don’t wait but call your doctor. It might need urgent medical care.

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

Your doctor gives you the prescribed medicine to save your life and effectively treat your condition. In addition, you probably got food and behavior recommendations. Once you know what the possible side effects and risks of the medication you take are, you can better take care and prevent those risks: (24)

  • Avoid NSAIDs painkillers (ibuprofen or naproxen) that 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 prolongs the time for clot-forming;
  • 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, swimming, and biking with a helmet. (24)


Despite your disease’s severity and side effects risk, you can put your life back to normal. 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 and as long 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.

Choose being informed and alert instead of frightened and insecure. 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.


1. Heart Disease facts, Center for Disease Control and Prevention, October 14, 2022,

2. Stroke facts, Center for Disease Control and Prevention, October 14, 2022,

3. Impact of Blood Clots on the United States, Centers for Disease Control and Prevention, June 9, 2022,

4. Blood Clots, MedlinePlus, April 21, 2021,

5. Blood clots: The good, the bad, and the deadly, Harvard Medical School, April 1, 2012,

6. Blood thinners, MedlinePlus, January 31, 2022,

7. C.M.M. de Jong et al., Incidence and impact of anticoagulation-associated abnormal menstrual bleeding in women after venous thromboembolism, Blood, October 20, 2022,

8. Menstrual Bleeding Worsens after Starting Blood Thinners, American Society of Hematology press release, August 29, 2022,

9. K.Pundi et al., Blood Thinners for Atrial Fibrillation Stroke Prevention, American Heart Association Journal, June 11, 2021,

10. Wilbs, J., Kong, XD., Middendorp, S.J. et al. Cyclic peptide FXII inhibitor provides safe anticoagulation in a thrombosis model and in artificial lungs. Nat Commun 11, 3890 (2020).

11. Understanding blood thinners, Harvard Medical School, January 1, 2020,

12. Heparin, MedlinePlus, September 15, 2017,

13. Warfarin, MedlinePlus, June 15, 2017,

14. Effient (prasugrel) tablets Medication Guide, March 28, 2019,

15. Eliquis (apixaban) tablets, Medication Guide, December 2012

16. PRADAXA (dabigatran etexilate mesylate) capsules, Medication Guide December 2011

17. Effient (prasugrel) tablets Medication Guide, March 28, 2019

18. XARELTO (rivaroxaban) tablets, Medication Guide, November, 2012

19. Stroke Signs and Symptoms, Center for Disease Control and Prevention, May 4, 2022,

20. Side effects of apixaban, NHS, May 14, 2022

21. Side effects of rivaroxaban, NHS, March 15, 2022

22. Anticoagulants, Cleveland Clinic, October 1, 2022,

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

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?

About the Author

Biljana Srbinovska
Biljana is a Master of Pharmacy, with a Healthcare Management specialization. Over 20 years of professional engagement to enable access to innovative treatments for patients in need. Biljana is dedicated to upgrading health education evidence-based, promoting a healthy lifestyle, and embedding healthy habits.

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