Who should not take blood thinners? A guide to contraindications

October 11, 2025 •

5 min read

While blood thinners are essential for preventing dangerous blood clots, their use carries significant bleeding risks, particularly in vulnerable individuals. Knowing who should not take blood thinners is a critical consideration to ensure patient safety and prevent potentially fatal complications, requiring a careful, individualized medical evaluation.

Quick Summary

Anticoagulants are contraindicated for individuals with active bleeding, severe liver or kidney disease, recent major trauma, or recent surgery. The decision requires a careful risk-benefit assessment, considering factors like uncontrolled hypertension, bleeding disorders, and pregnancy.

Key Points

Absolute Contraindications: Active bleeding, recent major surgery (especially neuro/eye/spine), recent intracranial hemorrhage, and severe bleeding disorders strictly prohibit blood thinners.
High-Risk Medical Conditions: Severe liver or kidney disease impairs the body's clotting ability and makes blood thinners dangerous.
Uncontrolled Hypertension: High blood pressure that is not controlled significantly increases the risk of a dangerous brain hemorrhage.
Drug Interactions and Comorbidities: Concurrent use of other medications like NSAIDs or the presence of conditions like cancer or gastric ulcers can heighten bleeding risk.
Alternatives Exist: For patients who cannot take blood thinners, procedural options like WATCHMAN™ devices or vena cava filters may be available to manage clot risk.
Individualized Assessment: Every decision is a risk-benefit analysis tailored to the patient, with advanced age and fall risk requiring careful consideration.

Understanding the Risks of Anticoagulant Therapy

Blood thinners, or anticoagulants, are medications that prevent blood clots from forming and enlarging. While lifesaving for conditions like deep vein thrombosis (DVT), pulmonary embolism (PE), and atrial fibrillation (AFib), they significantly increase the risk of bleeding. For this reason, these medications are not suitable for everyone. Medical professionals carefully evaluate a patient's overall health and specific risk factors before prescribing them. This evaluation determines whether certain conditions or recent medical events pose an unacceptable risk of a major bleed, especially a dangerous internal hemorrhage.

Absolute Contraindications: When Blood Thinners Are Dangerous

Absolute contraindications are conditions where blood thinner use is strictly avoided due to an extremely high risk of severe, uncontrolled bleeding. These situations typically outweigh any potential benefit of preventing clots.

Active, Clinically Significant Bleeding: The most obvious contraindication is any current, active bleeding, such as a major gastrointestinal bleed or internal hemorrhage. Administering a blood thinner in this scenario would be life-threatening.
Recent Major Surgery or Trauma: Patients who have recently undergone major surgery, especially neurosurgery (brain), ocular (eye), or spinal surgery, or experienced a major trauma, are at high risk of significant bleeding from the surgical or injury site. The risk is particularly high within the first few days to weeks post-procedure, and resumption of therapy is carefully timed.
Acute Intracranial Hemorrhage: Any recent or acute bleeding within the skull (intracranial hemorrhage or ICH) is an absolute contraindication. The risk of further bleeding and brain damage is exceptionally high. Similarly, a history of cerebral aneurysm or vascular malformations in the brain can also preclude therapy.
Severe Bleeding Disorders (Coagulopathies): Conditions like hemophilia or severe thrombocytopenia (very low platelet count) interfere with the body's natural clotting process, making blood thinners profoundly dangerous. While mild bleeding disorders might be managed with caution, severe cases require specialized care.
End-Stage Liver Disease: The liver produces many of the clotting factors necessary for hemostasis. In end-stage liver disease, this function is severely impaired, and a patient may already be prone to bleeding. Administering blood thinners would further compromise their already fragile clotting system. While the clotting picture in liver disease is complex, the severe form is a clear contraindication.
Pregnancy (For Specific Agents): The anticoagulant warfarin (Coumadin) is known to cross the placenta and can cause birth defects and fetal bleeding. It is therefore contraindicated during pregnancy. For pregnant women needing anticoagulation, safer alternatives like heparin are used.

Relative Contraindications: A Matter of Clinical Judgment

Relative contraindications require a careful, case-by-case assessment of risk versus benefit. The healthcare provider must weigh the patient's individual risk of a major bleed against their risk of a dangerous clot.

Uncontrolled High Blood Pressure (Hypertension): Patients with severe, uncontrolled hypertension (often defined as >200/120 mmHg) are at increased risk of intracranial hemorrhage and should not start blood thinners until their blood pressure is managed.
History of Gastrointestinal Bleeding or Ulcers: Patients with a history of stomach ulcers, gastrointestinal (GI) bleeding, or conditions like diverticulitis face a higher risk of recurrent GI bleeding while on blood thinners.
Advanced Age with High Fall Risk: While a high risk of falling isn't an automatic contraindication, advanced age combined with significant frailty and frequent falls warrants a careful discussion. The risk of an internal head bleed from a fall must be balanced against the risk of stroke or other thrombotic events.
Severe Chronic Kidney Disease: While DOACs (Direct Oral Anticoagulants) and Warfarin can be used in some kidney disease patients, severe impairment requires careful dosage adjustments or may necessitate alternative treatments. Some DOACs are not recommended for patients with end-stage renal disease.
Concurrent Medications: The simultaneous use of other medications that increase bleeding risk, such as NSAIDs (ibuprofen, naproxen), antiplatelet agents (aspirin, clopidogrel), or certain antibiotics and antifungals, increases the risk of complications.

Comparing Risks: Warfarin vs. DOACs

Different types of blood thinners have varying safety profiles and contraindications. This table highlights some key differences between Warfarin and the newer Direct Oral Anticoagulants (DOACs), such as Apixaban (Eliquis) and Rivaroxaban (Xarelto).


Feature	Warfarin	Direct Oral Anticoagulants (DOACs)
Mechanism	Inhibits Vitamin K-dependent clotting factors.	Directly inhibits a specific clotting factor (e.g., Factor Xa or Thrombin).
Onset	Slow, requires days to reach full effect.	Rapid, reaches full effect within hours.
Monitoring	Requires regular blood tests (INR) to monitor levels.	Does not require routine monitoring.
Kidney Function	Can be used with caution in severe CKD, but dose is affected.	Most require dosage adjustment in CKD, some contraindicated in end-stage renal disease.
Reversibility	Reversible with Vitamin K or fresh frozen plasma.	Specific reversal agents are available for some DOACs.
Drug Interactions	Extensive, sensitive to diet and many medications.	Fewer interactions overall, but still possible.

Alternatives for High-Risk Individuals

For patients with conditions that make blood thinners too risky, alternative strategies can be employed, particularly for those with AFib or a history of DVT.

Left Atrial Appendage Occlusion: For AFib patients, a device such as the WATCHMAN™ can be implanted to seal off the left atrial appendage, where most stroke-causing blood clots originate. This can reduce stroke risk without long-term blood thinner use.
Vena Cava Filters: For patients with a DVT who cannot receive anticoagulants, a small filter can be placed in the vena cava (a large vein in the abdomen) to prevent clots from traveling to the lungs (PE). These are typically used temporarily.
Compression Stockings: For managing DVT, compression stockings can help prevent blood from pooling in the legs and reduce swelling.
Thrombolytics: In certain severe cases, such as a massive pulmonary embolism, stronger clot-busting drugs (thrombolytics) may be used under strict medical supervision, but these carry a high bleeding risk.

Conclusion: The Importance of Informed Decisions

The question of who should not take blood thinners is not a simple one, as it involves a delicate balance between preventing a life-threatening clot and causing a life-threatening bleed. Conditions ranging from active bleeding and severe organ dysfunction to a high risk of falls all factor into this complex decision. The availability of newer DOACs and alternative procedural options has broadened the treatment landscape, but each carries its own set of risks and limitations. Ultimately, the decision must be made by a qualified healthcare professional who can conduct a thorough evaluation of the patient's medical history, current health status, and overall risk profile. A frank and open discussion between the patient and their doctor is paramount to ensuring the safest and most effective course of treatment.

Frequently Asked Questions

If you have a history of stomach ulcers or gastrointestinal bleeding, blood thinners may be considered a relative contraindication. The decision depends on the severity and frequency of the bleeding, and your doctor will weigh the risk of a new bleed against the risk of a blood clot.

Yes, severe and uncontrolled high blood pressure is a significant risk factor for intracranial hemorrhage (bleeding in the brain) and is often a reason to avoid blood thinners until blood pressure is managed.

No, certain blood thinners like warfarin are not safe during pregnancy because they can harm the fetus and increase bleeding risk. Pregnant women who require anticoagulation are typically given safer alternatives like heparin.

Any head trauma while on blood thinners, even if seemingly minor, requires immediate medical attention and a full evaluation, including a CT scan. This is due to the significantly increased risk of intracranial bleeding.

The decision for older adults with a high fall risk requires a careful risk-benefit assessment by a healthcare professional. While falls increase the risk of head bleeds, a high stroke risk may still warrant anticoagulation.

Absolute contraindications are definite reasons not to use a medication due to extremely high and certain risks. Relative contraindications are conditions that require a careful, individualized risk-benefit assessment, where the medication might still be used with caution.

Yes, alternatives include procedural options like the WATCHMAN™ device for AFib patients or vena cava filters for DVT, as well as managing risk factors with compression stockings.

Severe or end-stage liver or kidney disease makes blood thinners very risky and they are often contraindicated. Less severe cases require careful consideration, dosage adjustment, and monitoring by a doctor.