Blood thinners (anticoagulants) do not really thin the blood; they reduce the blood’s ability to clot, so that a clot does not grow and new clots do not form [1]. There are three main groups: newer tablets such as apixaban and rivaroxaban, warfarin tablets, whose dose is adjusted with the INR blood test, and heparin injections such as enoxaparin [1, 2]. The most important points: take the medicine on time, do not stop it without your doctor’s advice, and know the signs of bleeding [4, 3].
Key points
- A blood thinner (anticoagulant) does not really thin the blood; it stops a clot from growing and new clots from forming [1].
- There are three main groups: newer tablets such as apixaban and rivaroxaban, warfarin tablets, whose dose is adjusted with the INR test, and heparin injections such as enoxaparin [1, 2].
- Take the medicine at the same time every day and never stop it on your own; stopping early raises the risk of a clot and stroke [4, 5].
- Painkillers such as ibuprofen, naproxen and diclofenac, and also aspirin, raise the risk of bleeding; check with your doctor or pharmacist before taking any new medicine [3, 4].
- With warfarin you do not need to give up green leafy vegetables; just keep the amount you eat steady [3].
- Bleeding that does not stop, vomiting or coughing up blood, black stools or red urine mean you need medical help straight away: call your local emergency number [4, 3].
What is a blood thinner and what does it do?
Anticoagulants are usually called “blood thinners”, but they do not thin the blood. They reduce the blood’s ability to clot: they stop an existing clot from growing, so that the body can slowly absorb it, and they lower the chance of a new clot forming [1].
Doctors prescribe these medicines to treat and prevent blood clots in the leg and pulmonary embolism, to prevent stroke in a type of irregular heartbeat called atrial fibrillation, and for some people with an artificial heart valve [3, 4]. All of these medicines can cause bleeding; this is why taking them correctly and knowing the signs of bleeding matter [1].
Aspirin and clopidogrel are “antiplatelet” medicines, not anticoagulants [3]. To stop a clot in the veins from coming back, guidelines prefer an anticoagulant over aspirin [9].
Types of blood thinners
Newer tablets
Apixaban, rivaroxaban, dabigatran and edoxaban (direct oral anticoagulants). They do not need regular blood tests to adjust the dose and cause less bleeding than heparin injections followed by warfarin [2]. Unlike warfarin, vitamin K does not affect how they work [4, 5]. For most people with a deep vein clot or pulmonary embolism, guidelines prefer these tablets over warfarin [9]. They are not suitable for people with an artificial heart valve or a high-risk form of antiphospholipid syndrome [4, 6].
Warfarin
The oldest anticoagulant tablet. Its effect is measured with the INR test, and the doctor adjusts the dose according to the result [3]. Other medicines, changes in diet and illnesses such as diarrhoea, an infection or a fever affect the INR [3]. Warfarin is used for people with an artificial heart valve or a high-risk form of antiphospholipid syndrome [3, 2]. Its effect lasts 2 to 5 days after it is stopped [3].
Heparin injections
Enoxaparin (a low-molecular-weight heparin) is injected under the skin of the abdomen, and standard heparin is mostly given in hospital, into a vein [1, 8]. Low-molecular-weight heparin is the medicine of choice in pregnancy [11] and is also used at the start of treatment, until warfarin takes effect [8].
Which tests? The INR and other tests
The INR shows how much more slowly than normal the blood clots on warfarin. For a deep vein clot or pulmonary embolism, the target is 2.5 (between 2 and 3) [3]. Guidelines prefer this range over a lower one [9]; an INR above 4 usually adds no benefit but raises the risk of bleeding [3].
In the first days, the INR is measured every day until it settles in the target range; after that, usually every 1 to 4 weeks [3]. Whenever you start or stop another medicine, or take your medicines irregularly, the INR should be checked sooner [3].
The newer tablets do not need regular tests to adjust the dose [2]. But before any anticoagulant is started, a blood count, kidney and liver tests and clotting tests are checked [2], because the dose of some of these medicines depends on how well the kidneys work [5, 7]. To read your blood test report, see How to read a CBC.
How do I take the medicine correctly?
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On time, every time
Take the medicine at fixed times every day. Take dabigatran in the morning and evening, about every 12 hours [6], and apixaban twice a day [4]. Take the 15 mg and 20 mg tablets of rivaroxaban with food, at the same time each day [5]. Renew your prescription before you run out, so that no dose is missed [5].
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With or without food?
Apixaban, dabigatran and edoxaban can be taken with or without food [4, 6, 7]; the 15 mg and 20 mg tablets of rivaroxaban must be taken with food [5]. Swallow dabigatran capsules whole with a full glass of water and never open, chew or crush them [6]. If you cannot swallow apixaban or rivaroxaban tablets, ask your doctor; these tablets can be crushed [4, 5].
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| Medicine | What should I do? |
|---|---|
| Warfarin | Take it as soon as you remember on the same day; do not double the dose the next day, and tell your doctor [3] |
| Apixaban (twice a day) | Take it as soon as you remember on the same day and carry on with the twice-daily schedule; do not take two doses at once [4] |
| Rivaroxaban 15 mg twice a day (first 3 weeks) | Take it straight away so that you take 30 mg in total that day; in this case two 15 mg tablets can be taken together [5] |
| Rivaroxaban once a day (10 or 20 mg) | Take it straight away on the same day; do not double the dose within one day [5] |
| Dabigatran | Take it as soon as you remember on the same day; if your next dose is less than 6 hours away, skip the missed dose; never take a double dose [6] |
| Edoxaban | Take it as soon as you remember on the same day and carry on with your usual schedule the next day; do not take a double dose [7] |
If you are not sure, contact your doctor or pharmacist before your next dose. The only situation in which two tablets are taken together is rivaroxaban 15 mg twice a day [5].
Interactions with other medicines and food
Some medicines raise the risk of bleeding, and some make a blood thinner weaker or stronger. Before starting or stopping any medicine, including over-the-counter medicines, vitamins and herbal products, check with your doctor or pharmacist [4, 3].
- Painkillers and anti-inflammatory medicines: ibuprofen, naproxen, diclofenac, mefenamic acid and celecoxib, and also aspirin, raise the risk of bleeding [3, 4]. For pain, ask your doctor which painkiller is right for you.
- Some antidepressants, such as sertraline, fluoxetine, citalopram and venlafaxine, also raise the risk of bleeding [3, 4].
- Antibiotics and antifungal medicines with warfarin: starting or stopping any antibiotic or antifungal medicine may change the INR; for example co-trimoxazole, metronidazole, ciprofloxacin, clarithromycin, erythromycin and fluconazole. At these times, have your INR checked sooner [3].
- With the newer tablets: the tuberculosis medicine rifampicin, the epilepsy medicines carbamazepine and phenytoin and the herbal remedy St John’s wort lower the amount of apixaban and rivaroxaban in the blood and should not be taken with them; the antifungal medicines ketoconazole and itraconazole and the HIV medicine ritonavir raise the amount of these medicines in the blood [4, 5].
Food and warfarin: green leafy vegetables and some vegetable oils contain vitamin K, and a lot of vitamin K weakens the effect of warfarin. You do not need to give up these foods: eat a balanced diet, keep the amount of vitamin K steady, do not suddenly eat large amounts of green leafy vegetables, and talk to your doctor before changing your diet [3]. Grapefruit juice and some herbal products also interact with warfarin [3].
These interactions also matter with cancer medicines; doctors can find the details in Drug interactions with oral anticancer drugs.
Signs of bleeding and what to do
With these medicines you may bruise more easily, and bleeding from a small cut may take longer to stop [4]. But take these signs seriously and straight away call your doctor or get medical help [4, 3]:
- severe bleeding, or bleeding that does not stop
- unexplained or long-lasting bleeding from the gums, frequent nosebleeds, or periods or vaginal bleeding heavier than usual
- red, pink or brown urine
- red stools, or black stools that look like tar
- coughing up blood, vomiting blood or vomit that looks like coffee grounds
- unexplained pain, swelling or joint pain
- headache, dizziness or weakness
If bleeding becomes severe, hospitals have medicines that reverse or reduce the effect of anticoagulants; for example vitamin K and a clotting-factor concentrate for warfarin, a specific antidote for dabigatran, and protamine for heparin [16, 15, 8].
Before dental work, surgery or an endoscopy
If you are due to have surgery, a tooth extraction, an endoscopy or any other invasive procedure, tell the doctor who prescribed your blood thinner and your dentist or surgeon several days in advance [4]. Never stop the medicine on your own; your doctor will tell you whether it should be stopped, from when, and when to restart it [4, 5].
- The newer tablets are usually stopped 1 to 2 days before the procedure and restarted once bleeding has stopped; the timing depends on the type of procedure and on kidney function [4, 6, 13].
- In a large study, warfarin was stopped 5 days before the operation and restarted within 24 hours after it [12]. The INR is measured just before the procedure; for some minor procedures, the doctor may not stop warfarin and may just keep the INR at the low end of the range [3].
- Most patients do not need replacement heparin injections (“bridging”) during this time. In the same large study, these injections did not prevent clots any better but did cause more bleeding [12, 10].
Pregnancy and breastfeeding
If you take a blood thinner and are planning a pregnancy or have become pregnant, tell your doctor as soon as possible [4].
- Warfarin must not be used in pregnancy (except for some women with a mechanical heart valve), and taking it in the first 3 months of pregnancy can cause birth defects [3].
- The newer tablets, such as apixaban, are not recommended in pregnancy [4].
- Low-molecular-weight heparin injections (such as enoxaparin) are the medicine of choice in pregnancy [11].
- When breastfeeding, heparins and warfarin are considered safe, but the newer tablets are not recommended [11, 4].
These medicines can make periods heavier; if your periods have become heavier than usual, tell your doctor [4].
When should I see a doctor urgently?
Call your local emergency number or go to an emergency department straight away if [4, 3]:
- you have severe bleeding or bleeding that will not stop
- you vomit blood, your vomit looks like coffee grounds, or you cough up blood
- your stools are black like tar or bloody, or your urine is red or brown
- you have a headache, dizziness or weakness
- you have fallen and hit your head [4, 6]
- you have taken more than the prescribed dose [5, 6]
Call your doctor straight away if you have unexplained bruising, bleeding gums, frequent nosebleeds or periods heavier than usual, or if you take warfarin and have diarrhoea, an infection or a fever [4, 3].
If you have signs of a new clot, such as swelling of one leg or sudden breathlessness, see Blood clots in the leg and lung.
Frequently asked questions
How long do I need to take a blood thinner?
It depends on why it was prescribed. For a deep vein clot or pulmonary embolism it is at least 3 months; after that, depending on what caused the clot, the medicine is stopped or continued for longer [2, 9]. Read the details in Blood clots in the leg and lung.
What should I not eat with warfarin?
Can I take ibuprofen with a blood thinner?
Are the newer tablets better than warfarin?
For most people with a deep vein clot or pulmonary embolism, guidelines prefer the newer tablets over warfarin; they cause less bleeding and do not need regular INR tests [9, 2]. But for an artificial heart valve, a high-risk form of antiphospholipid syndrome or severe kidney failure, warfarin or heparin is a better choice [4, 2, 5].
Is it safe to have a tooth taken out while on a blood thinner?
How do I inject enoxaparin?
Before the first injection, you should be taught how to inject [8]. Tips from the medicine’s label [8]:
- Lie down and pinch a fold of skin on your abdomen between your thumb and index finger; insert the whole length of the needle into the fold and hold the fold until the injection is finished.
- Do not push the air bubble out of the prefilled syringe.
- Change the injection site each time between the left and right, front and side of the abdomen; do not inject into bruised or scarred skin or through clothes.
- Do not rub the injection site afterwards, so that it bruises less.
VTE treatment doses and prescribing notes (for doctors)
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- Before starting: full blood count, kidney and liver function, PT and aPTT [2]; CrCl by the Cockcroft-Gault equation [7], and for rivaroxaban based on actual body weight [5].
- Premature discontinuation of any oral anticoagulant raises the risk of thrombosis; if it is stopped for a reason other than pathological bleeding or completion of the course, consider cover with another anticoagulant [4].
- Neuraxial (spinal or epidural) anaesthesia and spinal puncture: risk of spinal or epidural haematoma and long-term or permanent paralysis (boxed warning) [4, 5, 8].
- DOACs are not recommended with prosthetic heart valves, and dabigatran is contraindicated with mechanical valves; DOACs are also not recommended in triple-positive antiphospholipid syndrome [4, 6].
- Concomitant antiplatelet agents, NSAIDs, SSRIs or SNRIs and other anticoagulants raise the risk of bleeding [4, 3].
- Apixaban is not a substitute for UFH in the initial treatment of PE with haemodynamic instability or in patients who may need thrombolysis or embolectomy [4].
| Drug | Initiation | Continuation | Reducing the risk of recurrence (after initial treatment) | Kidney, liver and interactions |
|---|---|---|---|---|
| Apixaban | 10 mg orally twice daily for 7 days [4] | 5 mg twice daily [4] | 2.5 mg twice daily, after at least 6 months of treatment [4] | No renal dose adjustment is recommended, including end-stage renal disease on dialysis, but patients with CrCl below 15 or on dialysis were not enrolled in the trials; not recommended in severe hepatic impairment (Child-Pugh C); with combined P-gp and strong CYP3A4 inhibitors (ketoconazole, itraconazole, ritonavir), halve the 5 mg or 10 mg dose and avoid them with the 2.5 mg dose; avoid combined inducers (rifampicin, carbamazepine, phenytoin, St John’s wort) [4] |
| Rivaroxaban | 15 mg twice daily with food for 21 days [5] | 20 mg once daily with food, at the same time each day [5] | 10 mg once daily, with or without food, after at least 6 months of standard treatment [5] | CrCl below 15 mL/min: avoid; avoid in moderate or severe hepatic impairment (Child-Pugh B or C) or hepatic disease with coagulopathy; avoid with combined P-gp and strong CYP3A inhibitors or inducers [5] |
| Dabigatran | After 5 to 10 days of parenteral anticoagulation [6] | 150 mg twice daily if CrCl is above 30 [6] | 150 mg twice daily, after previous treatment [6] | CrCl 30 or below, or dialysis: no dosing recommendation; CrCl below 50 with a P-gp inhibitor: avoid co-administration; swallow capsules whole; contraindicated with mechanical heart valves [6] |
| Enoxaparin | Outpatient (DVT without PE): 1 mg/kg subcutaneously every 12 hours; inpatient (DVT with or without PE): 1 mg/kg every 12 hours or 1.5 mg/kg once daily at the same time each day [8] | With warfarin (usually started within 72 hours), for at least 5 days and until the INR is 2 to 3 [8] | — | CrCl below 30: 1 mg/kg once daily [8]; dose by actual body weight without a cap, including in obesity; routine anti-Xa monitoring is not suggested in renal impairment or obesity [10]; contraindicated with immune-mediated HIT in the past 100 days or circulating antibodies [8] |
| Warfarin | Usually 2 to 5 mg once daily, without a loading dose; can be started together with heparin [3] | Adjust the dose to a target INR of 2.5 (2 to 3); usual maintenance dose 2 to 10 mg daily [3] | INR 2 to 3 (strong recommendation) [9] | Overlap with heparin for 4 to 5 days and until the INR is in the therapeutic range; INR daily until stable, then usually every 1 to 4 weeks; monitor the INR more often in renal impairment [3] |
| Edoxaban | After 5 to 10 days of parenteral anticoagulation [7] | 60 mg once daily [7] | — | 30 mg once daily if CrCl is 15 to 50, body weight is 60 kg or less, or with certain P-gp inhibitors (in the Hokusai-VTE trial: verapamil, quinidine, or short-term azithromycin, clarithromycin, erythromycin, oral itraconazole or oral ketoconazole); not recommended in moderate or severe hepatic impairment [7] |
Doses are from the current FDA labels; check the full label and the patient’s condition before prescribing. In the extended phase, CHEST 2021 prefers reduced-dose apixaban or rivaroxaban [19].
Pregnancy and breastfeeding (ASH 2018). In acute VTE in pregnancy, LMWH over UFH (strong recommendation); in women on therapeutic-dose LMWH, scheduled delivery with prior discontinuation of anticoagulation is suggested; when breastfeeding, UFH, LMWH, warfarin, acenocoumarol, fondaparinux or danaparoid are safe and DOACs are not recommended (strong recommendation) [11]. Warfarin is contraindicated in pregnancy except in women with mechanical heart valves who are at high risk of thromboembolism; first-trimester exposure was associated with a pattern of congenital malformations in about 5% [3].
Extremes of body weight and renal impairment (NICE). In people weighing less than 50 kg or more than 120 kg, consider monitoring therapeutic levels. With CrCl 15 to 50: apixaban, rivaroxaban, or LMWH for at least 5 days followed by edoxaban or dabigatran (if CrCl is 30 or more), or LMWH or UFH with a VKA. With CrCl below 15: LMWH, UFH, or LMWH or UFH with a VKA [2].
Switching between drugs. From warfarin to apixaban or dabigatran when the INR is below 2, to rivaroxaban when it is below 3, and to edoxaban when it is 2.5 or less [4, 6, 5, 7]. From a DOAC to a VKA: overlap the DOAC and VKA until the INR is in the therapeutic range, rather than bridging with LMWH or UFH; measure the INR just before the next DOAC dose [10].
High INR without bleeding and resumption after bleeding (ASH 2018). With an INR above 4.5 and below 10 without clinically relevant bleeding, temporarily stop the VKA without vitamin K. After major bleeding in patients who need long-term anticoagulation, resuming oral anticoagulation within 90 days is suggested over stopping it permanently [10]. Thrombosis in cancer: Venous thromboembolism in cancer.
Reversal and management of bleeding (for doctors)
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| Drug | Action | Notes |
|---|---|---|
| Warfarin | Stop warfarin; give four-factor prothrombin complex concentrate (4F-PCC) and up to 10 mg of intravenous vitamin K; PCC is preferred over fresh frozen plasma [10] | 4F-PCC dose by pre-treatment INR and body weight (weight up to 100 kg): INR 2 to below 4: 25 units/kg (maximum 2,500 units); 4 to 6: 35 units/kg (maximum 3,500); above 6: 50 units/kg (maximum 5,000); give vitamin K at the same time; repeat dosing is not recommended [16] |
| Dabigatran | Idarucizumab 5 g intravenously (two vials of 2.5 g in 50 mL) [15] | Data on an additional 5 g are limited [15]; haemodialysis removes dabigatran, but clinical experience with it in bleeding is limited [6] |
| Apixaban, rivaroxaban, edoxaban | Stop the drug; PCC may be considered, although it has not been evaluated in clinical studies [4, 5, 7] | ASH 2018 did not prefer 4F-PCC or andexanet alfa over the other [10]; but in December 2025 the FDA concluded that the risks of andexanet alfa outweigh its benefits (in the ANNEXA-I trial, thrombosis 14.6% versus 6.9% and thrombosis-related death by day 30 2.5% versus 0.9%), and US sales ended on 22 December 2025 [17]; activated charcoal reduces the absorption of apixaban, and haemodialysis has little effect on it; after PCC, monitoring with PT, INR, aPTT or anti-Xa activity is not useful [4] |
| Enoxaparin | Protamine sulfate by slow intravenous injection: 1 mg per 1 mg of enoxaparin if it was given in the previous 8 hours; 0.5 mg per 1 mg if more than 8 hours have passed or a second dose is needed [8] | Anti-Xa activity is neutralised by about 60% at most; if at least 12 hours have passed since the last injection, protamine may not be needed; risk of severe hypotensive and anaphylactoid reactions [8] |
| Unfractionated heparin (UFH) | Protamine sulfate: each 1 mg neutralises at least 100 units of heparin; very slow intravenous injection over 10 minutes, no more than 50 mg [18] | Because heparin is cleared quickly from the circulation, the dose of protamine needed falls as time passes [18] |
Reserve aggressive reversal for life-threatening bleeding (high cost and risk of thromboembolic complications) [10].
Interruption before surgery and procedures (for doctors)
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Warfarin and bridging. In the BRIDGE trial (1,884 patients with atrial fibrillation; warfarin stopped 5 days before the procedure and resumed within 24 hours after it), no bridging was non-inferior to dalteparin 100 IU/kg twice daily for arterial thromboembolism (0.4% versus 0.3%) and reduced major bleeding (1.3% versus 3.2%; RR 0.41) [12]. ASH 2018 recommends against bridging with LMWH or UFH in patients at low to moderate risk of recurrent VTE, in favour of interrupting the VKA alone [10]; the 2022 ACCP guideline also makes a strong recommendation against heparin bridging in atrial fibrillation [14]. Measure the INR just before the procedure; in minimally invasive procedures, keeping the INR at the low end of the therapeutic range may allow anticoagulation to continue safely [3].
DOACs. In the PAUSE cohort (3,007 patients with atrial fibrillation taking apixaban, dabigatran or rivaroxaban), the DOAC was omitted for 1 day before a low-bleeding-risk procedure and 2 days before a high-bleeding-risk procedure, and resumed 1 day after a low-risk procedure and 2 to 3 days after a high-risk procedure, without bridging or coagulation testing; 30-day major bleeding was 1.35% (apixaban), 0.90% (dabigatran) and 1.85% (rivaroxaban), and arterial thromboembolism 0.16%, 0.60% and 0.37% [13]. ASH 2018 suggests against routine laboratory testing of DOAC effect before procedures as the sole method, except where the drug effect may be prolonged (renal impairment, interacting drugs), interruption cannot be confirmed (urgent procedures) or the procedure carries a very high bleeding risk [10].
Label timings. Stop apixaban at least 48 hours before procedures with a moderate or high bleeding risk and at least 24 hours before low-risk procedures [4]; rivaroxaban and edoxaban at least 24 hours [5, 7]; dabigatran 1 to 2 days (CrCl 50 or more) or 3 to 5 days (CrCl below 50), and longer before major surgery, spinal puncture or placement of a spinal or epidural catheter [6]. Restart once adequate haemostasis is established; these drugs act quickly [4, 5]. If the patient cannot take oral medicines after surgery, consider a parenteral anticoagulant [5].
Related links
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References
19 referencesShow all referencesClose the reference list
Numbers in the text, such as [1], refer to these sources. Sources with a “Study card” button are summarised in the site’s evidence library.
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- 14Perioperative Management of Antithrombotic Therapy: An American College of Chest Physicians Clinical Practice Guideline
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- 17Update on the Safety of Andexxa by AstraZeneca: FDA Safety Communication
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- 19Antithrombotic Therapy for VTE Disease: Second Update of the CHEST Guideline and Expert Panel Report
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