11.2 High-Risk Drug Classes & Prescribing Safety

High-Risk Drug Classes & Prescribing Safety

Prescribing safety relies on identifying high-risk medication categories—frequently designated by the acronym APINCH (Anticoagulants, Potassium/Insulin, Narcotics/Opioids, Chemotherapeutic/Immunosuppressive agents, and High-concentration sedatives). Errors involving these classes account for over $50\%$ of preventable drug-related deaths in hospitalized patients.

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Anticoagulation Prescribing & Reversal Protocols

Anticoagulants are vital for stroke prevention in atrial fibrillation and treatment of venous thromboembolism (VTE), but carry high risks of life-threatening hemorrhage.

Warfarin Management

• Mechanism: Inhibits Vitamin K Epoxide Reductase Complex 1 (VKORC1), blocking $\gamma$-carboxylation of Factors II, VII, IX, X, and proteins C/S.
• Monitoring: International Normalized Ratio (INR). Target INR is $2.0-3.0$ for AF, DVT, and PE, or $2.5-3.5$ for mechanical mitral heart valves.

Direct Oral Anticoagulants (DOACs)

• Direct Thrombin Inhibitor: Dabigatran etexilate (150 mg BD).
• Direct Factor Xa Inhibitors: Apixaban (5 mg BD), Rivaroxaban (20 mg OD with food), Edoxaban (60 mg OD).
• Renal Cut-offs:
  • Dabigatran: Contraindicated if $\text{eGFR} < 30 \text{ mL/min}$.
  • Apixaban: Standard dose 5 mg BD. Reduce to 2.5 mg BD if patient meets $\ge 2$ of: Age $\ge 80$ years, Body weight $\le 60 \text{ kg}$, Serum creatinine $\ge 133 \ \mu\text{mol/L}$. Contraindicated if $\text{eGFR} < 15 \text{ mL/min}$.
  • Rivaroxaban/Edoxaban: Dose reduction required if $\text{eGFR } 15-49 \text{ mL/min}$; avoid if $\text{eGFR} < 15 \text{ mL/min}$.

Clinical Table: Emergency Anticoagulation Reversal Protocols

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Insulins and Inpatient Glycemic Safety

Insulin errors represent $15\%$ of high-alert hospital medication safety incidents, caused primarily by confusion between generic names, incorrect concentrations, or illegible dosage abbreviations.

• Prescribing Mandates:
  • Always prescribe insulin by brand name (e.g., Lantus, Toujeo, Abasaglar, Novorapid, Humalog, Actrapid). Different formulations of insulin glargine (e.g., Lantus 100 units/mL vs Toujeo 300 units/mL) are non-interchangeable on a unit-for-unit basis.
  • Write "units" in full. Never use "U" or "IU", which are easily misread as a zero (0) or number four (4), resulting in tenfold overdose (e.g., 10 U read as 100 units).
• Inpatient Hypoglycemia Protocol ($\text{Blood Glucose} < 4.0 \text{ mmol/L}$):
  • Conscious patient: $15-20 \text{ g}$ fast-acting carbohydrate (e.g., $200 \text{ mL}$ orange juice or 4-5 glucotabs), followed by complex carbohydrate.
  • Unconscious / NPO / Severe: IV access present $\rightarrow 75-100 \text{ mL}$ of 20% Glucose IV over 15 minutes (or $150-200 \text{ mL}$ 10% Glucose). If no IV access $\rightarrow$ IM Glucagon 1 mg.
• Oral Antidiabetic Safety:
  • Metformin: Hold prior to iodinated contrast media if $\text{eGFR} < 45 \text{ mL/min}$. Discontinue permanently if $\text{eGFR} < 30 \text{ mL/min}$ due to risk of metformin-associated lactic acidosis (MALA).
  • Sulfonylureas (Gliclazide): Accumulate in renal impairment, causing severe, prolonged hypoglycemia requiring extended observation ($>24 \text{ hours}$) and IV glucose infusions.

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Aminoglycoside Prescribing: Extended-Interval (Once-Daily) Dosing

Once-daily gentamicin ($5-7 \text{ mg/kg}$) utilizes concentration-dependent bactericidal killing ($\text{Peak}/\text{MIC} \ge 8-10$) and the post-antibiotic effect (PAE), while protecting the renal cortex and inner ear through an extended low-trough washout period.

• Dosing Weight: Calculate dosage using Ideal Body Weight (IBW). In obese patients ($\text{BMI} \ge 30 \text{ kg/m}^2$), use Adjusted Body Weight: $$\text{Adjusted Weight} = \text{IBW} + 0.4 \times (\text{Actual Weight} - \text{IBW})$$
• Monitoring: Measure serum concentration 18–24 hours after the first dose. Plot on Hartford nomogram or ensure trough level is $< 1.0 \text{ mg/L}$ prior to the second dose.
• Exclusions from Once-Daily Dosing: Infective endocarditis (requires synergistic low-dose gentamicin $1 \text{ mg/kg}$ BD/TID), severe burns, ascites, pregnancy, or $\text{eGFR} < 20 \text{ mL/min}$. Use conventional multiple-daily dosing in these settings.

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Immunosuppressant Safety: Methotrexate and Azathioprine

Methotrexate Prescribing Standards

• Frequency: Prescribed ONCE WEEKLY on a designated day of the week (e.g., every Monday) for rheumatoid arthritis, psoriasis, or inflammatory bowel disease. Daily dosing leads to fatal bone marrow suppression and gastrointestinal mucositis.
• Folic Acid Supplementation: Prescribe Folic Acid $5 \text{ mg}$ once weekly on a different day (e.g., Friday) to minimize mucosal and hematological toxicity without attenuating therapeutic efficacy.
• Pre-treatment Screening: Baseline FBC, LFTs, Renal function, and Chest X-ray (to exclude baseline pulmonary fibrosis).
• Drug Interactions: NSAIDs, co-trimoxazole, and penicillins reduce renal tubular excretion of methotrexate, precipitating acute toxicity.
• Toxicity Rescue: Calcium folinate (leucovorin) is the specific rescue agent for methotrexate toxicity.

Azathioprine & Allopurinol Interaction

• TPMT Screening: Test thiopurine S-methyltransferase (TPMT) activity before initiating azathioprine or 6-mercaptopurine. Homozygous TPMT deficiency ($0.3\%$ of population) causes profound aplastic anemia.
• Major Allopurinol Interaction: Allopurinol and febuxostat inhibit xanthine oxidase, the enzyme responsible for inactivating 6-mercaptopurine. Co-administration increases active 6-thioguanine nucleotides by 4-fold to 5-fold. If co-administration is unavoidable, the azathioprine dose MUST be reduced by 75% ($1/4$ of original dose) with weekly FBC monitoring.

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Opioid Equianalgesic Dosing & Safety

Opioid prescribing requires strict calculation of equianalgesic ratios to prevent accidental fatal hypoventilation.

Breakthrough Pain Calculation

• The breakthrough dose of immediate-release opioid equals $1/10\text{th}$ to $1/6\text{th}$ of the total 24-hour baseline oral morphine equivalent dose, administered every 2 to 4 hours as needed.

Opioid Rotation Ratios & Cross-Tolerance

• When switching between opioids, calculate the total daily oral morphine equivalent, apply the conversion factor, and then reduce the calculated dose by 30% to 50% to account for incomplete cross-tolerance.
• Transdermal Fentanyl Patch: Oral morphine $60-100 \text{ mg/24h} \approx \text{Fentanyl patch } 25 \ \mu\text{g/h}$ (changed every 72 hours). Patches are unsuitable for acute unstable pain due to slow onset ($12-24 \text{ hours}$ to steady state).

Renal Impairment & Opioid Selection

• Morphine & Codeine: Metabolized to morphine-3-glucuronide (M3G, neurotoxic causing myoclonus and hyperalgesia) and morphine-6-glucuronide (M6G, potent opioid agonist causing severe sedation and respiratory depression). Both accumulate rapidly when $\text{eGFR} < 30 \text{ mL/min}$.
• Safe Opioids in CKD Stage 4-5 ($\text{eGFR} < 30 \text{ mL/min}$): Fentanyl, Alfentanil, and Buprenorphine. These agents produce inactive metabolites and do not depend on renal excretion.

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Exam Pearls

MRCPI Exam Pearl: Co-prescribing Allopurinol and Azathioprine without reducing the azathioprine dosage by $75\%$ is a classic MRCPI safety scenario. Allopurinol blocks xanthine oxidase, leading to catastrophic bone marrow aplasia and life-threatening pancytopenia.

MRCPI Exam Pearl: In patients with severe renal impairment ($\text{eGFR} < 30 \text{ mL/min}$), morphine accumulation causes respiratory arrest due to M6G retention. The opioid of choice for severe pain in advanced CKD or dialysis is Fentanyl or Alfentanil.