Which common medications taken by seniors have clinically significant interactions with repeated-dose ivermectin?

1. Blood thinners: warfarin and the bleeding risk

Multiple drug references and regulatory guidance flag warfarin as a clinically important interaction with ivermectin—reports indicate ivermectin can increase warfarin’s effects, which raises the risk of bleeding and requires close monitoring of INR if co‑administered ^{[1] [2] [6] [7]}.

2. CYP3A4 inhibitors: drugs that raise ivermectin levels

Strong inhibitors of the CYP3A4 enzyme—commonly prescribed antifungals and some macrolide antibiotics such as ketoconazole and erythromycin—can increase systemic ivermectin exposure and therefore the chance of dose‑related toxicity; drug interaction lists repeatedly identify these classes as major to moderate interaction concerns ^{[1] [3]}.

3. P‑glycoprotein substrates and transporters: hidden elevating effects

Ivermectin is a substrate for drug transporters including P‑glycoprotein and ABC transporters, and agents that inhibit P‑gp (or drugs like erdafitinib that interact via P‑gp) can meaningfully raise ivermectin concentrations—Medscape flags erdafitinib as an agent to avoid co‑administration, illustrating that transporter interactions can be as clinically significant as metabolic ones ^{[4] [6]}.

4. Anticonvulsants and enzyme inducers: reduced efficacy risk

Some older anticonvulsants, exemplified by phenytoin, are enzyme inducers that may lower ivermectin plasma levels and potentially reduce its effectiveness; clinical resources list anticonvulsants as interacting drugs that can alter ivermectin’s activity ^[3].

5. Central nervous system depressants and symptom amplification

Case reports and interaction summaries note that combinations with sedating agents—including some benzodiazepines named in patient‑facing summaries (lorazepam, clonazepam)—and alcohol can increase central nervous system effects such as drowsiness or confusion, a particular concern in seniors who are more susceptible to falls and cognitive effects ^{[8] [2] [9]}.

6. Scope, prevalence, and practical advice for clinicians

Interaction databases enumerate over 100 possible ivermectin interactions with one source breaking them into one major, many moderate, and several minor alerts, which underscores that while a few interactions (warfarin, CYP3A4 inhibitors, P‑gp modulators, certain anticonvulsants) are most clinically important for older patients, the full list is broad and individualized review is necessary; additionally, age‑related liver, kidney, and heart disease common in older adults may require dose adjustments or extra monitoring ^{[1] [10] [5]}.

7. Caveats, uncertainty, and what the sources do not prove

The available clinical monographs and databases synthesize published interactions and pharmacology but do not always quantify absolute risk for every drug combination in routine geriatric care; some sources note only potential or theoretical mechanisms (transporter or CYP interactions) rather than large clinical trials demonstrating harm, and clinicians should interpret database flags in the context of patient comorbidities and monitoring capacity ^{[4] [11]}.

8. Bottom line for prescribers and pharmacists

For seniors receiving repeated ivermectin doses, prioritize review of concurrent anticoagulants (especially warfarin), strong CYP3A4 inhibitors (ketoconazole, erythromycin), P‑gp interacting oncology drugs (eg, erdafitinib), enzyme‑inducing anticonvulsants, and sedatives/alcohol; when co‑administration cannot be avoided, arrange laboratory monitoring (INR for warfarin), consider dose timing or alternatives, and document counseling given higher vulnerability from age‑related organ decline ^{[2] [3] [6] [5] [9]}.