NR 568 Week 3 Clarithromycin Interaction Analysis Example

Drug Interaction Analysis: Clarithromycin Added by an Urgent Care Clinic to a Settled Ten-Drug Regimen, What It Moves, How Far, How Soon and What Happens When It Stops

Student Name

College of Nursing, Chamberlain University

NR 568: Advanced Pharmacology for the Adult-Gerontology Primary Care Nurse Practitioner

Instructor Name

Month Day, Year

Drug Interaction Analysis: Clarithromycin Added by an Urgent Care Clinic to a Settled Ten-Drug Regimen, What It Moves, How Far, How Soon and What Happens When It Stops

The New Agent Described First

On a Friday evening, a week of sinus pain took the patient to an urgent care clinic, which sent the patient home with ten days of clarithromycin at 500 mg every 12 hours. Clarithromycin is a macrolide antibiotic and a strong inhibitor of CYP3A4 and of P-glycoprotein, and it prolongs the QT interval. The patient's regimen, reviewed two weeks earlier, includes atorvastatin 40 mg, citalopram 20 mg (reduced from 40 mg last month), trazodone 100 mg, doxazosin 4 mg, tamsulosin 0.4 mg, chlorthalidone 25 mg, lisinopril 40 mg, metformin 500 mg twice daily, allopurinol and levothyroxine. The estimated creatinine clearance is about 33 mL/min, which also matters because clarithromycin itself is partly cleared by the kidney, and the usual dose should be halved when clearance is below 30 mL/min.

Direction Before Significance

For each pairing, the analysis states which drug moves and in which direction before deciding whether it matters.

Atorvastatin moves up. Atorvastatin is metabolized by CYP3A4. Clarithromycin blocks that pathway, so atorvastatin levels rise, roughly several-fold at this dose, raising the risk of muscle injury and, rarely, rhabdomyolysis with kidney injury. In a population-based cohort of older statin users whose statin depends on CYP3A4, clarithromycin was associated with more hospitalizations for acute kidney injury and hyperkalemia and with higher mortality than azithromycin (Patel et al., 2013).

The QT interval moves longer. Clarithromycin prolongs the QT interval. So does citalopram, even at 20 mg, with an effect that rises with dose in records data (Castro et al., 2013), and trazodone to a lesser degree. Chlorthalidone can lower potassium and magnesium, which adds to the risk. The patient's last corrected QT was 468 milliseconds at 40 mg of citalopram.

Trazodone moves up. Trazodone is metabolized by CYP3A4, so its levels rise, adding sedation and orthostatic hypotension at bedtime, when doxazosin is also taken.

Tamsulosin moves up. Tamsulosin is metabolized partly by CYP3A4, and strong inhibitors raise its levels, adding to the orthostatic risk.

How Long It Takes to Arrive

Clarithromycin's inhibition of CYP3A4 begins within the first day or two and builds over the first several doses, because it is a mechanism-based inhibitor that inactivates the enzyme. The rise in atorvastatin and trazodone therefore arrives over the first two to four days. The QT effect is present from the first doses. When clarithromycin stops, the enzyme has to be resynthesized, so the inhibition fades over several days rather than at once; the interaction outlasts the last tablet by about three to five days.

Pairings Screened Out

Not every pairing on the screening report deserved action, and the analysis records why. Clarithromycin and metformin: no meaningful pharmacokinetic interaction at these doses. Clarithromycin and lisinopril or chlorthalidone: no direct interaction, though any illness with poor intake can raise kidney risk, which is watched through the usual sick-day advice. Clarithromycin and levothyroxine: no meaningful effect. Clarithromycin and allopurinol: no meaningful interaction. Writing these down shows the screen was read, not printed, and keeps attention on the pairings that move something.

Why the Clinic Chose a Macrolide

The urgent care clinician likely chose clarithromycin because the patient mentioned a remote rash with amoxicillin many years ago. The practice reviewed that history: a mild, nonitchy rash in childhood without hives or swelling. That is a low-risk history, and amoxicillin-clavulanate was given with a first-dose observation at the clinic. A recorded allergy that has not been assessed can push prescribers toward drugs with more interactions, which is a reason to clarify allergy labels as part of medication review.

Three Mechanisms, Three Answers

Metabolic (atorvastatin, trazodone, tamsulosin). The answer here is to avoid the inhibitor or hold the affected drug. Holding atorvastatin for the course and for five days afterward removes the muscle risk with little cardiovascular cost over two weeks. Trazodone can be halved for the same period.

Electrical (QT). The answer is to avoid stacking. With citalopram and trazodone in place and a borderline QT, a QT-prolonging antibiotic is a poor choice.

Clinical. The answer to the whole problem may be a different antibiotic or no antibiotic. Most acute sinusitis is viral, and bacterial sinusitis is usually treated, when treatment is needed, with amoxicillin-clavulanate rather than a macrolide, because pneumococcal macrolide resistance is common (Chow et al., 2012).

The Decision

The practice called the patient on Monday. The patient had taken three doses. Clarithromycin was stopped. Because symptoms had lasted more than 10 days with facial pain and purulent discharge, amoxicillin-clavulanate 500/125 mg every 12 hours for 5 to 7 days was prescribed, a dose adjusted for a clearance near 30 mL/min that avoids the high-dose 875 mg tablet. Atorvastatin was held for five days after the last clarithromycin dose, and trazodone was halved for the same five days. Potassium and magnesium were checked, and an electrocardiogram was performed at the visit.

Read It Backwards Too

The interaction also has to be read in reverse. When clarithromycin stops, atorvastatin and trazodone levels fall back over several days. If the trazodone dose had been raised during the course to address insomnia, the patient would become undertreated afterward. If atorvastatin had been continued, its levels would fall back toward normal. In this case, holding atorvastatin and halving trazodone, then restoring them on a set date, avoids both problems.

What a Removal Would Move

The analysis also asks what removing an existing drug would do. If the planned removal of doxazosin happens soon, the orthostatic risk from tamsulosin and trazodone falls. If trazodone is removed later in the deprescribing plan, the QT and sedation stacks shrink. The interaction profile is not fixed; it changes with each entry that leaves.

What the Patient and Urgent Care Need to Know

The patient was given a wallet card listing citalopram, atorvastatin and trazodone as drugs that interact with macrolide and azole antifungal medicines, with a request to show it at any clinic. A note was sent to the urgent care clinic explaining the change, without criticism, because the clinic did not have the current medication list.

The practice also added clarithromycin, erythromycin and the azole antifungals to the alert list in the patient's electronic record, so that any future prescription inside the health system triggers a warning on screen before the order is signed.

References

Castro, V. M., Clements, C. C., Murphy, S. N., Gainer, V. S., Fava, M., Weilburg, J. B., Erb, J. L., Churchill, S. E., Kohane, I. S., Iosifescu, D. V., Smoller, J. W., & Perlis, R. H. (2013). QT interval and antidepressant use: A cross sectional study of electronic health records. BMJ, 346, f288. https://doi.org/10.1136/bmj.f288

Chow, A. W., Benninger, M. S., Brook, I., Brozek, J. L., Goldstein, E. J. C., Hicks, L. A., Pankey, G. A., Seleznick, M., Volturo, G., Wald, E. R., & File, T. M., Jr. (2012). Executive summary: IDSA clinical practice guideline for acute bacterial rhinosinusitis in children and adults. Clinical Infectious Diseases, 54(8), 1041-1045. https://doi.org/10.1093/cid/cir1043

Patel, A. M., Shariff, S., Bailey, D. G., Juurlink, D. N., Gandhi, S., Mamdani, M., Gomes, T., Fleet, J., Hwang, Y. J., & Garg, A. X. (2013). Statin toxicity from macrolide antibiotic coprescription: A population-based cohort study. Annals of Internal Medicine, 158(12), 869-876. https://doi.org/10.7326/0003-4819-158-12-201306180-00004