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Prescription Writing — Format & Parts (Pharmacology)

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What is a Prescription?

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Parts of a Prescription

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1. Prescriber Information (Superscription Header)

• Prescriber's name, degree, address, and phone number
• This identifies the authorized person issuing the prescription
• In institutional settings, this may be a pre-printed letterhead

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2. Patient Information

• Patient's name, address, age, and weight (especially important for pediatric/geriatric dosing)
• Date the prescription was written
• This ensures the right drug reaches the right patient and enables age/weight-appropriate dosing

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3. The Superscription — "Rx"

• The symbol Rx (from Latin recipe, meaning "take thou")
• Appears at the top-left of the prescription body
• Signals the start of the drug order

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4. The Inscription (Body of the Prescription)

• The name of the drug (generic or brand)
• The dosage form (tablet, capsule, solution, ointment, etc.)
• The strength/concentration (e.g., 500 mg, 250 mg/5 mL)
• This is the core of the prescription — what drug and how strong

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5. The Subscription

• Instructions to the pharmacist on how to prepare/dispense the medication
• Includes:
  • Quantity to dispense (e.g., "Disp: #30 tablets" or "Dispense 100 mL")
  • Any compounding instructions if needed (e.g., "Make 30 capsules")
• May include: "Label" — directs the pharmacist to label the container

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6. The Signa (Sig) — Directions to the Patient

• Latin signa = "write" (i.e., write on the label)
• Instructions the pharmacist writes on the bottle label for the patient to follow:
  • Route of administration (oral, topical, etc.)
  • Dose (e.g., "Take 1 tablet")
  • Frequency (e.g., "twice daily," "every 8 hours")
  • Special instructions (e.g., "Take with food," "Avoid sunlight," "Do not crush")
• Example: "Sig: Take 1 tablet orally twice daily with food for 10 days"

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7. Prescriber's Signature & Registration

• Prescriber's signature (handwritten or electronic)
• DEA registration number — required for controlled substances (Schedule II–V drugs)
• Some jurisdictions require a state license number
• Space for "Substitution permitted" vs. "Dispense as written (DAW)" — indicates whether the pharmacist can substitute a generic equivalent
• Refill instructions: e.g., "Refill × 3" or "No refill" — Schedule II drugs cannot be refilled; Schedule III–V allow limited refills

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Rational Prescribing — The 6-Step Process

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Dosing Regimen Considerations (Step 5 Detail)

• Route of administration: oral is preferred for convenience; IV for emergencies or poor oral bioavailability
• Dose: determined by pharmacokinetics and the therapeutic window
• Frequency (dosing interval): ideally once or twice daily to improve compliance — patients taking a drug once daily are far more compliant than those taking it four times daily
• Duration of therapy: must be specified to avoid under- or over-treatment
• Loading dose: used when a rapid steady-state is needed (e.g., digoxin, amiodarone)

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Common Prescription Abbreviations

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Poor Prescription Writing — Common Errors to Avoid

1. Illegible handwriting — a leading cause of medication errors
2. Incomplete information — missing dose, frequency, duration, or route
3. Use of dangerous abbreviations — e.g., "U" for units (can be misread as "0"), ".5 mg" instead of "0.5 mg" (missing leading zero)
4. No indication of generic substitution — can lead to unnecessary cost to patient
5. Failure to specify refills — especially critical for controlled substances
6. Missing DEA number for controlled substances
7. Not counseling the patient — resulting in non-compliance or misuse

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Legal Requirements (Key Points)

• Drugs are classified as OTC (over-the-counter) or Rx Only (prescription required)
• Only licensed prescribers (physicians, dentists, podiatrists, veterinarians, and in many states nurse practitioners, physician assistants, pharmacists) can write prescriptions
• Schedule II controlled substances (e.g., morphine, oxycodone, amphetamines): require a separate, non-refillable prescription with DEA number
• Schedule III–V substances: allow refills (Schedule III–IV up to 5 refills in 6 months)
• Electronic prescriptions are now legally accepted in most jurisdictions and are mandatory for controlled substances in many US states

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Compliance and the Prescription

• Average patient receives 12.3 prescriptions/year
• Compliance improves with: fewer daily doses, patient education, simple packaging (blister packs), routine timing (e.g., with meals), refill reminders
• Prescriptions should always include clear patient instructions to minimize non-compliance

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Refill — Meaning

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In Simple Terms

• The pharmacist can give the patient that medication 3 more times after the first dispensing
• The patient does not need to revisit the doctor for a new prescription each time
• Total supply = 1 original + 3 refills = 4 times dispensed

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Why Refills Exist

• For chronic conditions (hypertension, diabetes, asthma) where long-term medication is needed
• Avoids unnecessary repeat doctor visits for stable, ongoing therapy
• Saves time and cost for the patient

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Refill Rules by Drug Schedule (USA)

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On the Prescription Label

• "Refill × 2" — can be refilled 2 more times
• "NR" or "No Refill" — pharmacist cannot dispense again without a new prescription
• "Refill PRN" — refill as needed (used for some chronic medications)

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Key Point

DEA — Drug Enforcement Administration

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What It Does

• Enforces the controlled substances laws and regulations of the USA
• Regulates the manufacture, distribution, and dispensing of legally produced controlled substances
• Combats illegal drug trafficking and abuse

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DEA Registration Number

Format of a DEA Number:

• 2 letters + 6 digits + 1 check digit
• Example: AB1234563
• First letter = type of registrant (e.g., A or B = doctor/practitioner, M = mid-level practitioner)
• Second letter = first letter of the prescriber's last name
• This number must appear on every prescription for a controlled substance

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Drug Schedules Under DEA (Controlled Substances Act)

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DEA Number on Prescriptions — Rules

• Mandatory only when prescribing Schedule II–V controlled substances
• Not required for regular (non-controlled) medications
• Pharmacists use it to verify the legitimacy of the prescription
• Forging or misusing a DEA number is a federal crime

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Relevance Outside the USA

• The DEA is a US-specific agency
• Other countries have equivalent bodies:
  • India → Narcotics Control Bureau (NCB) + state drug controllers
  • UK → Home Office regulates controlled drugs
  • International → International Narcotics Control Board (INCB) under the UN

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Key Point for Pharmacology Exams

A DEA number is required on a prescription only for controlled substances (Schedules II–V). For Schedule II drugs, even with a DEA number, no refills are permitted — a fresh prescription is needed each time.

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╔══════════════════════════════════════════════════════════════╗
║           DR. ANIL SHARMA, MBBS, MD (General Medicine)       ║
║           Reg. No.: MCI-2019-45678                           ║
║           City Health Clinic, 14 Main Street, New Delhi      ║
║           Phone: +91-11-2345-6789                            ║
╠══════════════════════════════════════════════════════════════╣
║                                                              ║
║  Patient Name : Mr. Rahul Mehta          Age: 28 yrs / M    ║
║  Address      : 22, Rose Garden, New Delhi                   ║
║  Date         : 05 / 05 / 2026                               ║
║  Weight       : 70 kg                                        ║
║                                                              ║
╠══════════════════════════════════════════════════════════════╣
║                                                              ║
║  Rx                                                          ║
║                                                              ║
║  Tab. Paracetamol 500 mg                                     ║
║  Disp. × 10 tablets                                          ║
║                                                              ║
║  Sig: Take 1 tablet orally SOS (as needed for fever /        ║
║       when temperature exceeds 38°C).                        ║
║       Do not take more than 1 tablet every 6 hours.          ║
║       Do not exceed 4 tablets in 24 hours.                   ║
║                                                              ║
║  Duration: 3–5 days or until fever resolves.                 ║
║                                                              ║
║  Special Instructions:                                       ║
║  • Take with water, preferably after food.                   ║
║  • Avoid alcohol during therapy.                             ║
║  • Do not combine with other paracetamol-containing          ║
║    products (e.g., cold/flu syrups).                         ║
║                                                              ║
╠══════════════════════════════════════════════════════════════╣
║                                                              ║
║  Refill: NR (No Refill)                                      ║
║                                                              ║
║  Substitution: Generic substitution permitted ✓              ║
║                                                              ║
║  Signature: Dr. Anil Sharma          [SEAL]                  ║
║                                                              ║
╚══════════════════════════════════════════════════════════════╝

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Parts Identified in This Prescription

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Why "No Refill" here?

Why No DEA Number?

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Pharmacogenomics — Complete Notes

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1. Definition & Overview

• Genomic medicine = how genes affect health
• Precision medicine (Personalized medicine) = tailoring care to the individual patient
• Pharmacogenomics = a branch of precision medicine that examines the impact of genetic variation on drug response
• Goal: Better predict how an individual will respond to a drug based on their genetics, so drug therapy can be customized

Traditional prescribing uses a "one size fits all" approach — same drug, same dose for all patients. But the same drug and dose can produce a wide range of responses in different individuals — some respond well, some get toxicity, some get no effect.

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2. Key Terminology

Allele

• Variant forms of a gene (e.g., B for brown eyes, b for blue eyes)
• In pharmacogenomics, alleles are reported using star (*) nomenclature (e.g., *1, *2, *4, *17)
• A single star allele may represent one SNP (single nucleotide polymorphism) or a group of variations (haplotype)
• *1 = wild type (no identified variation = normal function)

Genotype

• An individual's collection of genes at a locus
• In pharmacogenomics, reported as two alleles (e.g., *1/*4 or *2/*17)

Phenotype

• The observable/predicted trait arising from the genotype
• Derived by evaluating the functional status of each allele

Functional Status of Alleles

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3. Metabolizer Phenotypes (for Drug-Metabolizing Enzymes)

For drug transporters, phenotypes are: Poor/low function, Decreased function, Normal function, Increased function.

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4. Key Pharmacogenomic Resources

• CPIC has identified >250 drugs with pharmacogenomic implications
• >40 medications have the highest level of evidence (Level A) for changing prescribing based on genetics
• CPIC guidelines do not address who or when to test — only what to do when results are available

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5. Important Genes & Drug Examples

A. CYP2D6 — Most Studied Pharmacogenomic Gene

• Metabolizes ~25% of commonly used drugs
• Located on chromosome 22

⚠️ Classic exam case: A breastfeeding mother who is a CYP2D6 Ultrarapid Metabolizer takes codeine → excessive morphine excreted in breast milk → infant death. This led to black box warnings.

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B. CYP2C19 — Second Major Enzyme

Clinical case example from Lippincott: A 62-year-old post-MI patient with CYP2C19 genotype *2/*17 is classified as an Intermediate Metabolizer — *17 (increased function) does NOT overcome *2 (no function). Clopidogrel should be avoided; ticagrelor continued.

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C. TPMT (Thiopurine S-Methyltransferase)

• Metabolizes thiopurine drugs: azathioprine, 6-mercaptopurine (6-MP), thioguanine
• Used in leukemia, inflammatory bowel disease, organ transplant

• CPIC recommends TPMT (or NUDT15) testing before starting thiopurines
• NUDT15 is another gene — especially important in Asian populations for thiopurine toxicity

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D. DPYD (Dihydropyrimidine Dehydrogenase)

• Metabolizes fluoropyrimidines: 5-fluorouracil (5-FU), capecitabine (used in cancer)
• DPYD deficiency (poor metabolizer) → drug accumulates → severe, life-threatening toxicity (mucositis, neutropenia, neurotoxicity)
• Pre-treatment DPYD genotyping is now recommended in Europe

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E. HLA Alleles — Hypersensitivity Reactions

HLA-B*57:01 testing before abacavir is now standard of care worldwide.

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6. Types of Genetic Variants in Pharmacogenomics

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7. Prodrugs vs. Active Drugs — Critical Concept

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8. Pharmacogenomics Testing

• Germline testing (blood/saliva DNA) — tests inherited variants; done once in a lifetime
• Somatic testing (tumor DNA) — for cancer pharmacogenomics (e.g., EGFR mutations guiding gefitinib use)
• Pre-emptive genotyping: Testing before a drug is ever prescribed; results stored in the patient's chart for future use
• Reactive genotyping: Testing only when a specific drug is being considered

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9. Clinical Applications Summary

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10. Exam High-Yield Points

1. "One size fits all" → replaced by pharmacogenomics-guided precision prescribing
2. Star (*) nomenclature is used; *1 = normal/wild type
3. CYP2D6 — most clinically significant; affects ~25% of drugs
4. Prodrug + Poor Metabolizer = Therapeutic failure (e.g., codeine, clopidogrel)
5. Active drug + Poor Metabolizer = Toxicity (e.g., SSRIs, TCAs)
6. HLA testing is for immune reactions, not enzyme activity
7. TPMT deficiency → thiopurine toxicity → bone marrow suppression
8. Abacavir + HLA-B*57:01 positive = absolute contraindication
9. CPIC Level A = strongest evidence to change prescribing based on genetics
10. Pre-emptive genotyping = test once, use for lifetime of prescribing decisions

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