Testosterone Enanthate and Warfarin Interaction: What Patients and Clinicians Need to Know

What the testosterone enanthate label says about warfarin

The anticoagulant entry sits in the drug interactions section of the testosterone enanthate intramuscular prescribing information (effective May 2026), under the heading 'When administered concurrently, the following drugs may interact with androgens' [1]. It states that C-17 substituted derivatives of testosterone, such as methandrostenolone, have been reported to decrease the anticoagulant requirement. It also states that patients receiving oral anticoagulant therapy require close monitoring especially when androgens are started or stopped [1].

The sentence about a lower anticoagulant requirement names methandrostenolone as its example. It does not name testosterone enanthate [1].

The label's adverse reactions section adds a second relevant line: 'Suppression of clotting factors II, V, VII, and X; bleeding in patients on concomitant anticoagulant therapy' [1]. Warfarin inhibits the vitamin K epoxide reductase complex and lowers synthesis of factors II, VII, IX and X [2].

What the warfarin label says about adding or stopping a drug

The warfarin information here comes from the Jantoven warfarin sodium oral label, effective July 2025 [2].

Warfarin's boxed warning is bleeding risk, and it says to perform regular INR monitoring in all treated patients [2]. The label also says to perform more frequent INR monitoring when starting or stopping other drugs, including botanicals, or when changing their doses [2]. It does not name testosterone in that instruction.

The label sets a target INR of 2.5 (range 2.0 to 3.0) for venous thromboembolism and non-valvular atrial fibrillation. It notes that an INR above 4.0 appears to provide no additional therapeutic benefit in most patients and is associated with a higher risk of bleeding [2]. Bleeding is more likely in the first month of therapy. The named risk factors include INR above 4.0, age 65 or older, highly variable INRs, prior gastrointestinal bleeding, hypertension, cerebrovascular disease, anemia, malignancy, trauma, renal impairment, certain genetic factors, certain other drugs and long duration of therapy [2]. The label says that keeping the INR in the therapeutic range does not eliminate the risk of bleeding [2].

Does CYP2C9 explain the interaction?

The warfarin label says the more potent S-enantiomer is cleared mainly by CYP2C9 and R-warfarin by CYP1A2 and CYP3A4, and that CYP2C9 inhibitors can raise INR by increasing warfarin exposure [2]. Its table of example CYP2C9 inhibitors includes the anabolic steroid oxandrolone, and testosterone is not in that table [2]. The testosterone enanthate label does not name a CYP enzyme or any other mechanism for its anticoagulant entry [1].

How much does the warfarin dose change?

Neither label gives a percentage dose change or an expected INR shift for this combination [1][2]. The warfarin label says dosing is individualized and adjusted to the INR response, and that genotype can inform initial dose selection. It calls for daily INR determinations on initiation until stable in the therapeutic range, then every 1 to 4 weeks [2]. The enanthate label reports that C-17 substituted testosterone derivatives may decrease the anticoagulant requirement [1].

What to watch for

The warfarin label says to instruct patients about measures that minimize bleeding risk and to report signs and symptoms of bleeding. It lists fatal and nonfatal hemorrhage from any tissue or organ as the most common adverse reactions [2]. The testosterone enanthate label's own adverse reaction list includes bleeding in patients on concomitant anticoagulant therapy, alongside suppression of clotting factors II, V, VII and X [1].