The main medications that cause blood clots are estrogen-containing birth control pills and hormone replacement therapy, tamoxifen and raloxifene, certain cancer drugs such as thalidomide, lenalidomide, and asparaginase, testosterone, erythropoiesis-stimulating agents, glucocorticoids, some antipsychotics, and JAK inhibitors. Heparin can paradoxically trigger clots through heparin-induced thrombocytopenia. For most people the added risk is small, but it rises sharply when combined with other risk factors such as surgery, immobility, cancer, smoking, or an inherited clotting disorder.
In my practice, patients often ask whether a prescribed drug is safe for them. The answer usually depends less on the drug alone and more on the whole picture of their risk, which this guide will help you understand.
How Medications Tip the Clotting Balance
Normal blood flow depends on a balance between factors that form clots and factors that dissolve or prevent them. Doctors describe three drivers of clotting, known as Virchow’s triad: slow or stagnant blood flow, damage to the vessel wall, and blood that is more prone to clot (hypercoagulability).
Most clot-promoting drugs act on the third element. Estrogen, for example, increases the liver’s production of several clotting factors and lowers natural anticoagulants. Other drugs damage the vessel lining or activate platelet aggregation, the clumping of platelets that starts a clot. People with existing clotting disorders are especially vulnerable.
Common Medications Linked to Blood Clots
| Drug group | Examples | Main clot type |
|---|---|---|
| Combined hormonal contraceptives | Estrogen-progestin pills, patch, vaginal ring | Venous (DVT, pulmonary embolism) |
| Menopausal hormone therapy | Oral estrogen with or without progestin | Venous; also stroke |
| Selective estrogen receptor modulators | Tamoxifen, raloxifene | Venous |
| Myeloma drugs | Thalidomide, lenalidomide, pomalidomide (especially with dexamethasone) | Venous |
| Other cancer therapies | Asparaginase, cisplatin, bevacizumab | Venous and arterial |
| Erythropoiesis-stimulating agents | Epoetin, darbepoetin | Venous and arterial |
| Hormones and steroids | Testosterone, glucocorticoids | Venous |
| Antipsychotics | Clozapine, olanzapine, and others | Venous |
| JAK inhibitors | Tofacitinib, baricitinib, upadacitinib | Venous and arterial |
| Heparin (paradoxical) | Unfractionated heparin, less often low-molecular-weight heparin | Venous and arterial (HIT) |
Hormonal Therapies
Estrogen is the best-known culprit. The risk is highest in the first months of use, in smokers over 35, and in women with an inherited condition such as factor V Leiden. Progestin-only pills, hormonal IUDs, and transdermal estrogen for menopause carry lower clot risk than oral estrogen.
Cancer Treatments
Cancer itself raises clot risk, and some treatments add to it. The immunomodulatory drugs used in multiple myeloma are a classic example, which is why patients taking them routinely receive aspirin or an anticoagulant as prevention. Asparaginase, used in acute lymphoblastic leukemia, lowers the body’s natural anticoagulant antithrombin.
Heparin-Induced Thrombocytopenia
Heparin-induced thrombocytopenia (HIT) is an immune reaction in which antibodies activate platelets. It typically appears 5 to 10 days after starting heparin, with a platelet fall of more than 50%. Despite the low platelet count, it causes clots rather than bleeding, and heparin must be stopped immediately and replaced with a non-heparin anticoagulant.
Antipsychotics and JAK Inhibitors
Some antipsychotics are associated with venous clots, possibly through sedation, weight gain, and effects on platelets. JAK inhibitors, used for rheumatoid arthritis and other inflammatory conditions, carry regulatory warnings about clots, particularly in older patients with cardiovascular risk factors.
Who Is Most at Risk?
Drug-related clots rarely come from one factor alone. Risk is highest when a clot-promoting medication is combined with:
- A personal or family history of blood clots
- Inherited thrombophilia, such as factor V Leiden or prothrombin gene mutation
- Recent surgery, trauma, or hospitalization
- Long periods of immobility, including long-haul travel
- Active cancer, obesity, smoking, or pregnancy
- Older age
These factors add together. A woman taking a combined pill who also smokes, carries factor V Leiden, and is immobilized after an injury faces a far higher risk than any one of those factors would suggest. That is why I always ask about family history before recommending estrogen-based treatment, and why hospitals assess clot risk for every admitted patient.
For people who have already had an unprovoked clot, many of the drugs in the table above are avoided altogether, or used only with an anticoagulant alongside them.
Recognizing the Symptoms
A deep vein thrombosis (DVT) usually causes swelling, pain, warmth, and redness in one leg, often the calf. A pulmonary embolism may cause sudden shortness of breath, sharp chest pain worse on breathing, a fast heart rate, or coughing up blood. Arterial clots cause stroke symptoms (face drooping, arm weakness, speech difficulty) or heart attack symptoms.
Diagnosis uses ultrasound for DVT and CT pulmonary angiography for pulmonary embolism. A D-dimer blood test helps rule out a clot when suspicion is low.
Prevention and Management
Before starting a higher-risk drug, your doctor should review your clot history and family history. Depending on the situation, options include choosing a lower-risk alternative, adding preventive aspirin or an anticoagulant, or pausing the medication around surgery.
If a clot does occur, treatment is anticoagulation, typically with a direct oral anticoagulant, low-molecular-weight heparin, or warfarin, usually for at least three months. The medication that contributed is often stopped or switched. Staying active, avoiding smoking, and keeping well hydrated on long journeys all help.
When to See a Doctor
- Seek emergency care for sudden breathlessness, chest pain, coughing up blood, or stroke symptoms.
- See a doctor promptly for new one-sided leg swelling or pain.
- Tell your prescriber before starting a hormonal or high-risk drug if you or a close relative has had a clot.
- Do not stop a prescribed medication on your own; discuss safer alternatives first.
Frequently Asked Questions
Which birth control has the lowest clot risk?
Progestin-only methods, such as the progestin-only pill, hormonal IUD, and implant, carry little or no added clot risk. Copper IUDs contain no hormones at all. Combined estrogen methods carry the highest risk.
Can ibuprofen or other painkillers cause blood clots?
NSAIDs such as ibuprofen are linked mainly to a small increase in heart attack and stroke risk, especially at high doses and with long-term use. They are not considered a major cause of venous clots. People with heart disease should discuss regular NSAID use with their doctor.
Does testosterone therapy cause blood clots?
Testosterone can raise the red blood cell count, which thickens the blood, and has been associated with venous clots. Doctors monitor the hematocrit during treatment and may lower the dose if it rises too high.
Should I stop my medication if I am worried about clots?
No, not without advice. Stopping some drugs suddenly can be harmful. Talk to your prescriber, who can weigh your personal risk and, if needed, switch you to a safer option.
