9.2 Interactions, Contraindications, and Monitoring

The current TCO's Implementation domain asks you to select and deliver a pharmacotherapeutic intervention and to recognize interactions and contraindications. Planning chose the class. Implementation is the moment you either sign the script or refuse the pair. Evaluation (Chapter 19) later asks whether the drug worked and whether the labs moved. This section is the refusal-and-monitoring chapter.

Absolute versus relative contraindications

An absolute contraindication means do not give the drug in this situation — the expected harm is unacceptable. Any nitrate plus any PDE5 inhibitor (sildenafil, tadalafil, vardenafil, avanafil) is the cleanest FNP example: cGMP piles up, vessels collapse, and the patient can arrest. This is not a "take them six hours apart" problem. If a man on tadalafil develops chest pain, the emergency team needs to know the PDE5 drug is on board so they do not reach for nitroglycerin. Other absolute patterns you should treat as never-start: abacavir after a positive HLA-B*5701, isotretinoin in pregnancy, ACE inhibitor or ARB in the second and third trimester (and generally avoided throughout pregnancy), and live vaccines in pregnancy (Section 9.3).

A relative contraindication means the drug is usually a bad idea and you need a documented reason, extra monitoring, or a safer alternative — not that the molecule is magically banned from earth. Combined oral contraceptives in a 36-year-old who smokes are a relative (and, at 35-plus with 15 or more cigarettes, effectively treated as do-not-use) risk for thrombosis. A statin in compensated chronic liver disease may still be used with a plan. AGS Beers Criteria live in this relative box: they list drugs that are potentially inappropriate in older adults (strong anticholinergics, sliding-scale insulin as the sole regimen, chronic benzodiazepines, first-generation antihistamines). Beers is a risk list, not a statute. If you keep a Beers drug, write why, use the lowest dose, and name the review date. If a safer option exists, switch.

High-yield pairs you should refuse or intensively monitor

Warfarin deserves a second look because the stem often hides the antibiotic as a "simple UTI" or "dental prophylaxis." You do not need to memorize every INR-raising drug. You do need the reflex: new antimicrobial or azole plus warfarin means an INR plan this week, not at the next routine visit in two months. NSAIDs may not change the INR much and still bleed the gut.

Serotonin syndrome is easy to under-call because tramadol is marketed as a "non-opioid-ish" pain pill. It is still serotonergic and still a 2D6 prodrug. A patient already on sertraline who fills tramadol after a sprain is a classic Implementation miss. Triptans plus an SSRI are widely co-prescribed; the exam still wants you to recognize the stack and not add a third agent (tramadol, linezolid, methylene blue, another MAOI-like drug).

Baseline and follow-up labs that belong on the plan

ACE inhibitor or ARB. Check creatinine and potassium within 1–2 weeks of start or a material dose increase, then periodically. A small creatinine rise can be hemodynamic and acceptable; a large jump, hyperkalemia, or oliguria is not. Bilateral renal-artery stenosis and advanced CKD make the rise more dangerous. Combine this clock with the potassium-sparing pair above.

Statin. Obtain a baseline ALT before you start. Recheck ALT when the patient has symptoms of hepatotoxicity (fatigue, dark urine, jaundice, right-upper-quadrant pain) or when another drug or illness makes injury likely. Routine calendar rechecks in an asymptomatic adult are not the modern default — do not invent a mandatory six-week liver panel if the stem patient feels well and has no interacting drug. Recheck CK if unexplained muscle pain or weakness appears, especially after a CYP3A4 inhibitor is added to simvastatin or lovastatin.

Metformin and eGFR (FDA / ADA language). Obtain eGFR before the first dose and at least annually, more often if the kidney is at risk. Do not start metformin if eGFR is under 30 mL/min/1.73 m². Initiating metformin is not recommended when eGFR is 30–45. If the patient is already on metformin and eGFR falls into the 30–45 band, reassess benefit and risk, consider a lower dose, and watch the kidney more closely. Stop metformin if eGFR falls below 30. This replaced the older "creatinine 1.4 / 1.5" sex-based cutoff. Do not fail an item by quoting that retired number. Hold metformin around iodinated contrast when eGFR is reduced, then restart only after renal function is rechecked and stable.

Photosensitivity. Doxycycline and fluoroquinolones cause sunburn out of proportion to exposure. Counseling is Implementation, not decoration: clothing, shade, and sunscreen, and stop for blistering reactions. This is not a reason to withhold doxycycline for Rocky Mountain spotted fever; it is a reason to teach.

Isotretinoin is a specialty/REMS drug (iPLEDGE): documented negative pregnancy tests, two forms of contraception, monthly visits, and a closed pharmacy system. The FNP's job in primary care is to recognize teratogenicity, not to open an iPLEDGE account on a Friday afternoon for acne that has not failed safer therapy. If a patient of childbearing potential is already in iPLEDGE, do not start a conflicting teratogen or stop contraception "just for a month."

Beers Criteria without turning them into a ban

Use Beers to start a conversation, not to auto-cancel every amitriptyline that a pain clinic started a decade ago. Strong anticholinergics worsen cognition, constipation, urinary retention, and falls. Sliding-scale insulin without basal coverage is a hypoglycemia machine. Chronic benzodiazepines raise fall and delirium risk. If you continue a Beers drug, the note should say why (failed alternatives, short hospice horizon, specialist-directed) and what you will watch. If the stem offers a safer on-target alternative — topical rather than oral anticholinergic, acetaminophen rather than chronic NSAID in CKD, SSRI rather than amitriptyline for depression in a frail 86-year-old — take it.

Vignette. A 71-year-old with AF on warfarin, eGFR 38, and sertraline presents with dysuria. Reflex TMP-SMX is three mistakes at once: INR potentiation, hyperkalemia risk with an ACE inhibitor if one is on the list, and a folate hit if the patient were also on methotrexate. Nitrofurantoin may be wrong at this eGFR as well. Pick an antibiotic that fits the organism and the kidney, recut the INR, and do not add ibuprofen for the "ache."

Implementation items reward the FNP who can name the pair, name the harm, and name the lab date. If you cannot name all three, you are not ready to sign.