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Heparin: 7 things you should know

Medically reviewed by Carmen Fookes, BPharm. Last updated on May 26, 2022.

1. How it works

  • Heparin may be used to treat or prevent blood clots occurring in the blood vessels or lungs.
  • Heparin acts on multiple sites within the coagulation cascade to prevent blood from clotting and preexisting blood clots from becoming larger. Small amounts of heparin prevent the conversion of prothrombin to thrombin and large amounts prevent the conversion of fibrinogen to fibrin.
  • Heparin belongs to the class of medicines known as anticoagulants (may also be called a blood thinner).

2. Upsides

  • Heparin is an anticoagulant (stops blood from clotting) and is used to treat and prevent blood clots occurring in the blood vessels or lungs.
  • Guidelines recommend heparin as an appropriate choice for the treatment of acute proximal deep vein thrombosis or pulmonary embolism. Guidelines may differ in their recommendations for other heparin uses.
  • May be used for the initial treatment of acute venous thromboembolism in patients with renal impairment. For patients without renal impairment, LMWHs or fondaparinux are generally preferred.
  • Low-dose heparin may also be used before surgery to reduce the risk of blood clots. May be preferred by some guidelines for preventing postoperative DVT or PE in patients undergoing general (such as abdominal) surgery. May be considered in cardiac surgery patients with a complicated postoperative course; otherwise, mechanical prophylaxis is preferred in patients undergoing cardiac surgery.
  • May be used to prevent blood clots in other circumstances, such as during cardioversion of atrial fibrillation or flutter, or during pregnancy; however, LMWH is usually preferred during pregnancy because of a more favorable safety profile.
  • IV heparin may be preferred over subcutaneous (SC) treatments in people with PE in whom thrombolytic therapy is being considered or if there is concern about adequate SC absorption.
  • Small doses of heparin may be used to keep indwelling blood catheters patent (free from coagulated blood).
  • May be administered by IV infusion, intermittent IV injection, or deep subcutaneous injection. Should not be administered IM because of the frequency of hematomas at the site.
  • After heparin has been administered, an oral anticoagulant such as warfarin or an LMWH is generally administered for at least 3 months in adults with venous thromboembolism. Therapy is usually overlapped for a minimum of 5 days and until their INR is at least 2 for at least 24 hours.
  • Available in a preservative-free formulation which should be administered to neonates, infants, and also in pregnant or nursing women.
  • Generic heparin is available.

3. Downsides

If you are between the ages of 18 and 60, take no other medication or have no other medical conditions, side effects you are more likely to experience include:

  • Hemorrhage (unexpected, spontaneous bleeding), which is sometimes fatal, is the major risk with heparin use. Incidence is higher in women over the age of 60 years, and in those with certain medical conditions that increase their risk of hemorrhage. Protamine sulfate may be used by emergency department staff to neutralize the effects of heparin. Women may require lower dosages of heparin.
  • Heparin has been associated with several fatal medication errors. It is vital that health professionals use the correct dosage of heparin.
  • Must be given by injection.
  • May not be suitable for people with severe high blood pressure; bacterial endocarditis; about to undergo or just following major surgery, spinal taps, or spinal anesthesia; preexisting bleeding disorders; gastric ulceration; menstrual disorders; or liver disease.
  • May cause several other severe side effects including heparin-induced thrombocytopenia (increases the clumping together of platelets which can increase the risk of blood clot formation) and thrombocytopenia (a significant decrease in the number of platelets). Platelet counts and blood pressure may need monitoring.
  • May cause false elevations in liver enzymes (specifically AST and ALT). Long-term use has been associated with osteoporosis, hair loss, and priapism (sustained erections). Lipid levels may rise on discontinuation of heparin.
  • May interact with some drugs including aspirin, NSAIDs, and others that also affect blood clotting.
  • Laboratory monitoring, including coagulation tests, is usually required on heparin initiation; however, daily monitoring may not be required with fixed low-dose, long-term treatment.
  • Fatal medication errors have occurred as a result of confusion between different formulations of heparin.

Note: In general, seniors or children, people with certain medical conditions (such as liver or kidney problems, heart disease, diabetes, seizures) or people who take other medications are more at risk of developing a wider range of side effects. View complete list of side effects

4. Bottom Line

  • Heparin reduces the ability of the blood to clot and is used to treat or prevent blood clots. Heparin is given by injection and excessive bleeding is its main side effect. Laboratory monitoring (usually activated partial thromboplastin times [APTTs]) is required, at least initially, to determine effectiveness.
  • Plasma levels and APTTs of heparin may be higher in people over the age of 60 compared to younger patients, despite similar doses of heparin.

5. Tips

  • Heparin may make you bruise or bleed more easily. In addition, it may take longer than usual for you to stop bleeding after a cut. Report any significant bruising or bleeding to a doctor or emergency services if more appropriate.
  • Tell your dentist and other health professionals that you are receiving heparin before any surgery, no matter how minor, is scheduled.
  • Talk to your pharmacist or doctor before buying any over the counter medications, herbal remedies, or supplements, because some may contain ingredients that interact with heparin.
  • If you intend to become pregnant or breastfeed while you are on heparin; talk to your doctor first.
  • Call your doctor if you are on heparin and experience unexpected bleeding from your nose or if your periods are significantly heavier. Call emergency services if you have unexplained bleeding in your mouth or when you cough; following a bowel motion; or if you injure or cut yourself and can't stop the blood flow.
  • Tell your doctor if you are sensitive to heparin or pork products before heparin is administered.

6. Response and effectiveness

  • Heparin is usually given into a vein (intravenously), although it can be given by injection just under the skin (subcutaneously). Heparin rapidly reduces the ability of the blood to clot.
  • Heparin works immediately following direct IV injection or infusion. Works within 20 to 60 minutes following deep SC injection.
  • Initially, daily blood tests that measure activated partial thromboplastin time [aPTT] (a measure of how fast the blood is clotting) are required. However, daily blood tests are not usually required with low-dose, long-term heparin.

7. Interactions

Medicines that interact with heparin may either decrease its effect, affect how long it works for, increase side effects, or have less of an effect when taken with heparin. An interaction between two medications does not always mean that you must stop taking one of the medications; however, sometimes it does. Speak to your doctor about how drug interactions should be managed.

Common medications that may interact with heparin include:

  • antihistamines (may partially counteract effect)
  • apixaban
  • aspirin
  • corticosteroids, such as prednisone and methylprednisone
  • dabigatran
  • dextran (may increase risk of hemorrhage)
  • digoxin (may partially counteract effect)
  • dipyridamole
  • estrogen derivatives or progestins
  • herbs, such as alfalfa, ginkgo, or bilberry
  • hydroxychloroquine
  • methotrexate
  • mifepristone
  • nicotine
  • nitroglycerin
  • NSAIDs, such as ibuprofen, diclofenac, etodolac, and naproxen
  • omeprazole
  • potassium salts
  • ritonavir or tipranavir
  • tetracyclines
  • vitamin E.

In general, any medicine that can increase the risk of bleeding such as clopidogrel, SSRI antidepressants (eg, citalopram, duloxetine, fluoxetine, venlafaxine), fish oils, or other anticoagulants (such as warfarin) may interact with heparin.

Alcohol may increase the risk of stomach bleeding with heparin.

Note that this list is not all-inclusive and includes only common medications that may interact with heparin. You should refer to the prescribing information for heparin for a complete list of interactions.


Further information

Remember, keep this and all other medicines out of the reach of children, never share your medicines with others, and use heparin only for the indication prescribed.

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

Copyright 1996-2022 Revision date: May 25, 2022.