Testim Drug Interactions: What to Avoid and What to Tell Your Doctor

If you take Testim along with other medications, you need to know about potential drug interactions. Some interactions can affect how well Testim works, while others can increase your risk of side effects. Here's a complete guide to Testim drug interactions — what to avoid, what to monitor, and what your doctor and pharmacist need to know.

Always Tell Your Doctor and Pharmacist About All Your Medications

Before starting Testim, give your prescriber and pharmacist a complete list of everything you're taking — prescription medications, over-the-counter drugs, vitamins, supplements, and herbal products. Even substances that seem unrelated can affect how testosterone is metabolized or how it interacts with other drugs.

Most Important Testim Drug Interactions

1. Warfarin and Anticoagulants (Blood Thinners) — Monitor Closely

This is the most clinically significant Testim drug interaction. Androgens, including testosterone, can alter anticoagulant activity. If you take warfarin (Coumadin) or other blood thinners, Testim can increase the anticoagulant effect — meaning your blood thins more than expected, which can lead to excessive bleeding.

The FDA prescribing information for Testim specifically states that more frequent monitoring of INR (international normalized ratio) and prothrombin time is required for patients taking warfarin. Your anticoagulant dose may need adjustment when starting or stopping Testim.

What to do: Tell your doctor immediately if you take warfarin or any anticoagulant. Expect more frequent INR monitoring when starting Testim.

2. Insulin and Diabetes Medications — Blood Sugar May Drop

Androgens including testosterone can lower blood glucose levels by improving insulin sensitivity. If you take insulin or oral diabetes medications (like metformin, sulfonylureas, or other hypoglycemic agents), starting Testim could cause blood sugar to drop lower than expected.

What to do: Tell your doctor if you have diabetes. Monitor blood sugar more carefully when starting Testim, and report any unexplained low blood sugar episodes. Your diabetes medication doses may need adjustment.

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3. Corticosteroids (Prednisone, Methylprednisolone, etc.) — Increased Fluid Retention

Both testosterone and corticosteroids can cause sodium and water retention. Using them together can increase the risk of significant edema (fluid retention), particularly in patients with existing heart, kidney, or liver disease. The Testim prescribing information specifically warns that concurrent use with ACTH or corticosteroids should be done cautiously.

What to do: Tell your doctor if you take prednisone, methylprednisolone, dexamethasone, or any other corticosteroid. Monitor for ankle swelling, rapid weight gain, or shortness of breath.

4. Propranolol and Other Beta-Blockers

Studies with injectable testosterone have shown that testosterone may increase the clearance (elimination rate) of propranolol — a beta-blocker used for high blood pressure, heart rate control, and anxiety. If this effect applies to Testim, propranolol levels could drop, making it less effective. Note: The Testim prescribing information acknowledges this interaction but states it is "unknown" if it applies specifically to topical gel.

What to do: Tell your doctor if you take propranolol or other beta-blockers. Monitor blood pressure and heart rate response when starting Testim.

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5. Oxyphenbutazone — Elevated Drug Levels

Concurrent use of androgens and oxyphenbutazone (an anti-inflammatory) may result in elevated serum levels of oxyphenbutazone. Oxyphenbutazone is rarely used in the U.S. today, but if you use anti-inflammatory medications, mention them to your prescriber.

Testim Effects on Lab Tests

Testim can interfere with certain laboratory tests, even if it doesn't cause symptoms:

Thyroid function tests: Androgens decrease levels of thyroxin-binding globulin (TBG), resulting in decreased total T4 serum levels and increased T3 resin uptake. Free thyroid hormone levels remain normal — this is a lab artifact, not true thyroid dysfunction.

PSA (prostate-specific antigen): Testosterone can elevate PSA. Inform all providers and labs that you are on testosterone therapy.

Hematocrit/CBC: Testosterone stimulates red blood cell production, elevating hematocrit and hemoglobin. This is expected and requires periodic monitoring.

What to Tell All Your Healthcare Providers

Always mention Testim to:

Every doctor, nurse practitioner, or physician assistant who treats you

Your pharmacist when picking up any new prescription

Any lab technician conducting blood tests (it affects thyroid, PSA, and blood count results)

Any anesthesiologist or surgical team before a procedure (testosterone affects blood clotting)

The Bottom Line

The most important Testim drug interaction is with warfarin — if you take a blood thinner, tell your doctor before starting Testim and expect more frequent INR monitoring. Diabetes medications and corticosteroids are also worth discussing. For a full picture of Testim's side effect profile, see our guide to Testim side effects. Need help finding Testim at a pharmacy? Visit medfinder.