Disoproxil Fumarate: Interactions, Dosage, and Administration Guide

When you see Disoproxil Fumarate is a prodrug of tenofovir, an antiviral that blocks reverse transcriptase in HIV and suppresses hepatitis B virus replication in a prescription, you know the drug is designed to release the active molecule once it reaches your bloodstream. The most common brand name for Disoproxil Fumarate is Viread, and it’s been a backbone of antiretroviral therapy for over two decades.

What is Disoproxil Fumarate?

Disoproxil Fumarate (often abbreviated as TDF) belongs to the nucleotide reverse‑transcriptase inhibitor (NRTI) class. Chemically it is the fumarate salt of disoproxil, which improves oral bioavailability of tenofovir. After absorption, the disoproxil part is cleaved, releasing tenofovir diphosphate, the active form that competes with natural nucleotides during viral DNA synthesis.

Key attributes:

• Drug class: NRTI
• Approved for: HIV‑1 infection, chronic hepatitis B
• Typical oral dose: 300 mg once daily
• Half‑life: ~17 hours (parent), ~60 hours (active metabolite)

Pharmacology and Mechanism of Action

Once inside the cell, tenofovir is phosphorylated to tenofovir diphosphate. This metabolite mimics deoxy‑adenosine triphosphate and gets incorporated into the viral DNA chain. Because it lacks a 3′‑hydroxyl group, chain elongation stops, halting replication.

Tenofovir has a high affinity for HIV reverse transcriptase but a much lower affinity for human DNA polymerases, which explains its relatively good safety profile. However, the drug is cleared almost entirely by the kidneys via glomerular filtration and active tubular secretion, making renal function a pivotal factor in dosing.

Approved Indications and Typical Dosage Regimens

For HIV‑1 infection, Disoproxil Fumarate is usually paired with other antiretrovirals in a fixed‑dose combination (e.g., Tenofovir/Emtricitabine, Tenofovir/Emtricitabine/Efavirenz). The standard adult dose is 300 mg taken orally once daily, with or without food. In hepatitis B, the same dose is used, often as monotherapy or combined with lamivudine.

Special populations:

• Pediatrics: Children ≥2 years or ≥35 kg can take the adult dose; younger or lighter patients require weight‑based dosing (6 mg/kg).
• Renal impairment: Dose adjustments are required when creatinine clearance (CrCl) < 50 mL/min. For CrCl 30‑49 mL/min, reduce to 300 mg every 48 hours; for CrCl <30 mL/min, use 300 mg every 72 hours or consider switching to tenofovir alafenamide.

Administration Guidelines: How to Take Disoproxil Fumarate Correctly

1. Swallow the tablet whole with a full glass of water.
2. Take it at the same time each day to maintain steady plasma levels.
3. If you miss a dose, take it as soon as you remember unless it’s within 12 hours of the next scheduled dose; in that case skip the missed one.
4. Avoid crushing or chewing, as this can affect drug absorption.
5. Do not co‑administer with high‑dose vitamin D supplements without checking calcium levels, as bone mineral density can be affected.

When paired with protease inhibitors like ritonavir or cobicistat, the combined regimen may increase tenofovir plasma concentration, so closer monitoring of renal function is recommended.

Major Drug Interactions to Watch

Because Disoproxil Fumarate is cleared renally, any drug that alters kidney transporters can shift its levels.

• Proton pump inhibitors (PPIs) - Can reduce absorption slightly; spacing the dose by a few hours mitigates the effect.
• NSAIDs - Additive risk of kidney injury, especially in patients with pre‑existing renal disease.
• Didanosine - Combined use increases risk of pancreatitis.
• Lamivudine or Emtricitabine - Generally safe; the combo is standard in HIV therapy.
• Metformin - Both are excreted by the kidneys; monitor serum creatinine and adjust metformin dose if renal function declines.
• Ritonavir / Cobicistat - Boost tenofovir levels; consider dose reduction or switch to tenofovir alafenamide in high‑risk patients.

Always inform your clinician about over‑the‑counter supplements like calcium carbonate or herbal products, as they can also impact renal handling.

Monitoring, Safety, and Common Side Effects

Before starting therapy, obtain baseline labs:

• Serum creatinine and estimated glomerular filtration rate (eGFR)
• Bone mineral density (DEXA) for patients with osteoporosis risk
• HBsAg, HBeAg, and HIV viral load as disease‑specific markers

During treatment, repeat renal function tests every 3‑6 months. Watch for:

• Elevated serum creatinine (≥0.5 mg/dL rise from baseline)
• Proteinuria or phosphaturia indicating tubular dysfunction
• Decreased bone mineral density - especially in post‑menopausal women and older men
• Gastrointestinal upset (nausea, diarrhea) - usually mild and transient
• Rare lactic acidosis in patients with mitochondrial toxicity risk

If renal function falls below CrCl 30 mL/min, switching to the newer prodrug tenofovir alafenamide (TAF) is often safer due to lower systemic exposure.

Tenofovir Disoproxil Fumarate vs. Tenofovir Alafenamide: A Quick Comparison

Both drugs share the same mechanism of action, but TAF’s improved intracellular delivery allows a much lower dose, translating into better safety for kidneys and bones. Clinicians often start patients on TDF and switch to TAF if renal markers or bone density begin to decline.

Practical Tips for Patients and Providers

• Keep a medication diary - note any new OTC drugs or supplements.
• Stay hydrated; adequate fluid intake lessens tubular stress.
• Schedule lab work on the same day each month to catch trends early.
• Discuss lifestyle factors such as smoking or heavy alcohol use, which can compound renal risk.
• If you experience persistent nausea, try taking the tablet with food even though food isn’t required.

Open communication with your healthcare team is the best defense against hidden interactions and side effects.

Frequently Asked Questions

Understanding how Disoproxil Fumarate works, its dosing nuances, and its interaction profile empowers you to stay on therapy safely. Keep these guidelines handy, discuss any concerns with your prescriber, and you’ll maximize the benefit of this long‑standing antiviral.