What medications increase blood clots? A guide to thrombogenic drugs

October 12, 2025 •

5 min read

Estimates suggest as many as 900,000 Americans experience a blood clot each year, with some cases linked to common medications. Understanding what medications increase blood clots is crucial for patients and healthcare providers to manage risks effectively. Some drugs disrupt the body's natural balance of clotting factors, while others cause damage to blood vessel walls.

Quick Summary

Several drug classes, including hormonal contraceptives, certain cancer therapies, and some anti-inflammatory drugs, can elevate the risk of blood clot formation. Mechanisms range from increasing clotting factors to damaging vessel linings. This risk varies based on the specific medication, dosage, and individual patient risk factors.

Key Points

Hormonal Birth Control and HRT: Estrogen-containing contraceptives and hormone replacement therapy increase the risk of blood clots by affecting clotting factors and are riskier with higher doses or other risk factors.
Selective Estrogen Receptor Modulators (SERMs): Drugs like tamoxifen and raloxifene, used for cancer and osteoporosis, are also linked to an elevated risk of venous clots.
Cancer and Chemotherapy: Both the disease and specific treatments, such as cisplatin, L-asparaginase, and thalidomide, increase clotting risk through vessel damage and altered blood composition.
Heparin-Induced Thrombocytopenia (HIT): Paradoxically, the anticoagulant heparin can trigger an immune-mediated reaction leading to severe, systemic blood clotting.
Other Risks: NSAIDs, particularly COX-2 inhibitors, and some antipsychotic drugs have also been linked to increased risk of arterial and venous clots, respectively.
Risk Reduction: To minimize risk, discuss your medication history and personal risk factors with your doctor, stay active, and be aware of blood clot symptoms.
Never Stop Abruptly: Do not discontinue any prescribed medication without first consulting your healthcare provider, as abrupt cessation can be dangerous.

How Medications Influence Clotting

Blood clotting, or thrombosis, is a vital process to stop bleeding, but it can become dangerous if a clot forms inappropriately inside a blood vessel. Some medications can disrupt the delicate balance between pro-clotting and anti-clotting substances in the body, leading to a hypercoagulable state. The risk of developing a blood clot varies depending on the specific drug, dosage, and individual patient risk factors such as age, family history, and lifestyle.

Hormonal Therapies and Clotting Risk

Hormone-based medications are some of the most widely recognized for their association with an increased risk of blood clots. The estrogen component is a primary culprit, as it can increase the levels of certain clotting factors and lower the concentration of natural anticoagulants.

Oral Contraceptives (Birth Control Pills) and Hormone Replacement Therapy (HRT)

Combination oral contraceptives, and HRT used during and after menopause, contain estrogen, which is linked to a higher risk of venous thrombosis. The level of risk depends on the dose of estrogen and the type of progestin used. For most women, the overall risk remains low, but it increases significantly for those with other risk factors, such as a history of blood clots, smoking, or obesity. Newer pills with lower estrogen doses generally carry a lower risk, but pregnancy itself carries a higher risk of clots than hormonal birth control.

Selective Estrogen Receptor Modulators (SERMs)

Drugs like tamoxifen, used to treat breast cancer, and raloxifene, used for postmenopausal osteoporosis, act on estrogen receptors but can still increase the risk of venous thromboembolism (VTE). The mechanism is similar to hormonal therapies, affecting the balance of clotting factors and anticoagulants. Newer SERMs and aromatase inhibitors may present lower thrombotic risks than older versions or standard hormone therapy.

Testosterone and Anabolic Hormones

Testosterone and related anabolic compounds can increase blood viscosity and activate platelets, leading to an elevated risk for both arterial and venous thromboses. Cases of androgen-related clotting events have been documented, especially in individuals taking high doses for purposes like bodybuilding.

Cancer and Its Treatments

Both cancer itself and the therapies used to fight it can contribute to a hypercoagulable state, making patients particularly vulnerable to blood clots.

Chemotherapy Agents

Certain chemotherapy drugs, such as cisplatin, L-asparaginase, and thalidomide, are known to damage the endothelial lining of blood vessels, which is a major trigger for clot formation. Thalidomide, an immunomodulatory drug, is specifically associated with VTE, especially when combined with steroids. L-asparaginase can reduce the body's natural anticoagulants, further increasing risk.

Erythropoietin and Immunomodulators

Erythropoietin-stimulating agents, used to treat anemia associated with cancer or chronic kidney disease, can increase blood viscosity by boosting red blood cell production, thereby increasing thrombotic risk. Immunomodulatory drugs and certain immunotherapies can also increase clotting risk, though mechanisms vary by drug.

The Paradoxical Case of Heparin

It seems counterintuitive, but the anticoagulant heparin, used to prevent blood clots, can sometimes trigger a dangerous immune reaction known as heparin-induced thrombocytopenia (HIT).

Heparin-Induced Thrombocytopenia (HIT)

In HIT, the immune system produces antibodies against a complex of heparin and platelet factor 4 (PF4). This causes a drop in platelet count (thrombocytopenia) and paradoxically leads to an increased risk of severe, systemic thrombotic events. The condition requires immediate cessation of heparin and administration of alternative anticoagulants.

Other Medications with Clotting Risks

Beyond hormonal and cancer therapies, several other common medications carry a risk of increasing blood clots.

Nonsteroidal Anti-Inflammatory Drugs (NSAIDs)

NSAIDs, particularly older COX-2 inhibitors like rofecoxib (withdrawn from the market), have been linked to an increased risk of arterial thrombosis (heart attack, stroke). This occurs due to an imbalance between prostacyclin and thromboxane, favoring the latter and promoting clotting.

Antipsychotic Drugs

Some antipsychotic medications have been associated with an increased risk of venous thromboembolism, particularly with certain medications like clozapine. While the exact mechanism is not fully understood, potential factors include sedation leading to immobility and platelet activation.

Other Notable Medications

Antifibrinolytic Drugs: Medications like tranexamic acid, used to control bleeding, can prevent the natural breakdown of clots, increasing the risk of venous thrombosis.
Systemic Corticosteroids: Long-term use of systemic corticosteroids like prednisone can increase levels of pro-clotting factors, raising the risk of VTE.
Energy Drinks and Stimulants: High doses of stimulants in energy drinks, or illicit substances like cocaine, can induce platelet activation and endothelial damage, leading to thrombotic events.

Comparing Common Thrombogenic Medications


Medication Class	Examples	Primary Clotting Risk	Mechanism of Action
Hormonal Contraceptives	Combined birth control pills	Venous thrombosis (DVT/PE)	Increases clotting factors, decreases natural anticoagulants
Hormone Replacement Therapy	Estrogen-based therapies	Venous thrombosis (DVT/PE)	Increases clotting factors, decreases natural anticoagulants
SERMs	Tamoxifen, Raloxifene	Venous thrombosis (DVT/PE)	Estrogen-like effect on coagulation system
Cancer Treatments	Cisplatin, L-asparaginase, Thalidomide	Venous and arterial thrombosis	Endothelial damage, increased clotting factors
Heparin	All forms	Venous and arterial thrombosis	Triggers an autoimmune reaction (HIT)
NSAIDs	Older COX-2 inhibitors	Arterial thrombosis (MI/stroke)	Imbalance between prostacyclin and thromboxane
Erythropoietin	Epoetin	Venous and arterial thrombosis	Increases blood viscosity and platelet number

Minimizing Your Risk

If you are taking any medication associated with an increased risk of blood clots, discuss your personal risk profile with your healthcare provider. Prevention strategies may be necessary, especially if you have additional risk factors. The American Cancer Society recommends several preventive measures for at-risk individuals, including:

Reviewing your medications: Keep an updated list of all drugs and supplements, including over-the-counter options, to discuss with your doctor.
Managing other risk factors: Address modifiable risks like smoking and obesity.
Staying active: During long periods of sitting, such as travel or hospitalization, move and stretch your legs frequently to promote blood flow.
Knowing the symptoms: Familiarize yourself with the signs of a blood clot, such as pain, swelling, and redness in a limb, or chest pain and shortness of breath.
Discussing prophylaxis: For individuals at high risk, or during periods like hospitalization, your doctor may recommend anticoagulant medication or compression devices.

Conclusion

While many medications offer significant health benefits, it is important to be aware of their potential side effects, including the risk of blood clots. Hormonal therapies, certain cancer treatments, and even some anti-inflammatory drugs can interfere with the body's natural coagulation system. The risk is often manageable, especially with open communication with a healthcare team, awareness of individual risk factors, and adherence to preventive measures. Never stop or change a prescribed medication without consulting your doctor first. For more information on preventing blood clots, the CDC offers valuable resources on their website (https://www.cdc.gov/bloodclots/).

Frequently Asked Questions

Hormonal medications containing estrogen, such as combination birth control pills and hormone replacement therapy, are among the most common medications known to increase the risk of blood clots. The risk, while generally low for most people, is highest when starting the medication and is influenced by dosage and other personal risk factors like smoking or obesity.

Yes, many cancer treatments can cause blood clots. Chemotherapy agents like cisplatin and L-asparaginase, immunomodulatory drugs like thalidomide, and hormone therapies such as tamoxifen are all associated with an increased risk of both venous and arterial clots.

In a rare condition called heparin-induced thrombocytopenia (HIT), heparin can trigger an immune system response that forms antibodies against a complex of heparin and platelet factor 4. This reaction paradoxically causes severe, systemic blood clotting and requires the immediate cessation of heparin.

Not all NSAIDs carry the same level of risk. The risk of arterial thrombosis (such as heart attack or stroke) was most significantly linked to older selective COX-2 inhibitors that were developed to reduce stomach side effects. The mechanism involves an imbalance in chemicals that regulate clotting.

No, you should never stop taking a prescribed medication without first speaking to your healthcare provider. Your doctor can evaluate your specific risk factors, discuss alternative birth control methods if necessary, and advise you on the safest course of action.

Symptoms depend on the location of the clot. In the legs (Deep Vein Thrombosis or DVT), look for pain, swelling, or redness in one leg. A pulmonary embolism (PE), a clot in the lungs, can cause chest pain, shortness of breath, and a rapid heartbeat.

Other risk factors include obesity, cancer, smoking, older age, a personal or family history of blood clots, and conditions that cause prolonged immobility, such as long-distance travel or hospital stays.

Topical and lower-dose estrogen therapies are generally considered to carry a lower risk than higher-dose or oral forms. However, the risk is not eliminated, and the overall risk profile depends on the specific product and patient characteristics.