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Math and Dosage Calculations for Health Care Third Edition
Booth & Whaley
Chapter 11: Calculations for Special Populations
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Learning Outcomes
11.1 Explain why dosages for special populations must be based on theindividual patient.
11.2 Identify factors that affect the absorption, distribution,biotransformation, and elimination
of drugs in special populations.
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11.3 Determine safe doses for special populations.
11.4 Calculate patient dosages based on body weight.
11.5 Find a patient’s body surface area (BSA).
Learning Outcomes (cont.)
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11.6 Calculate patient dosages based on a patient’s BSA.
11.7 Describe volume and medication limitations for special populations.
11.8 Calculate infusion rates based upon body weight.
Learning Outcomes (cont.)
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Introduction
Two populations requiring extra consideration when calculating medication dosages Pediatric (less than 18 years old) Geriatric (over 65 years old)
Risk of harm is far greater due to the way they break down and absorb medications.
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Introduction (cont.)
Clarify all confusing drug orders
Calculate with absolute accuracy
Verify that dose is safeSeek assistance from your
supervisor
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Do not take short cuts with medication calculations.
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Factors That Impact DosingStandardized doses based on
assumptions The patient’s body and age Body systems are fully developed and
functioning
Assumptions do not hold true for all populations or situations
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Factors That Impact Dosing – PHARMACOKINETICS
Study of how drugs are used by the body Absorption Distribution Biotransformation Elimination
Understanding these processes allows for adjustments for special populations
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Factors That Impact Dosing – PHARMACOKINETICS (cont.)
Absorption Process that moves a drug from the
site where it is given into the bloodstream
IV medications bypass the absorption process
Oral medications – digestive system
Topical – through the skin
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Factors That Impact Dosing – PHARMACOKINETICS (cont.)
Distribution Process that moves the drug from the
bloodstream to other body compartments
Target site – where the drug produces its desired effect
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Factors That Impact Dosing – PHARMACOKINETICS (cont.)
Biotransformation Process that chemically changes the
drug in the body
Occurs primarily in the liver
Helps to protect the body from foreign chemicals including drugs
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Factors That Impact Dosing – PHARMACOKINETICS (cont.)
Elimination Process where the drug leaves the
body Main route – urine Other ways
Air that we exhaleSweatFecesBreast milkOther body secretions
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Factors That Impact Dosing – PHARMACOKINETICS (cont.)
Adjust dose If one of the four processes are not
functioning within certain limits
Made according to nature and severity of patient’s condition
May by higher or lower than standard doses
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Factors That Impact Dosing (cont.)
Conditions that impact dosing Stomach or intestinal disorders Liver disorders Obesity Kidney disease
Functions of body systems change over the life of a person.
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Factors That Impact Dosing (cont.)
Newborns – systems not fully developed pH of stomach is
lower Thinner skin Liver still
developing Less circulation to
muscles
Geriatrics – systems deteriorate Skin and veins
become fragile Decreased liver
function Decreased kidney
function Poor circulation
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Working with Special Populations
Other Considerations Parent or caretaker may be
administering or assisting with medications.
Educate regarding medications
Geriatric patients may have decreased awareness or understanding.
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Working with Special Populations – GERIATRIC PATIENTS
Show respect
Listen to their concerns
Encourage them to Use the same pharmacy to fill all
prescriptions Have one primary care physician to monitor
and approve all medications Keep a list of all medications
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Special Populations – GERIATRIC PATIENTS (cont.)
Decreased dexterity Injections Eye drops Open bottles
Difficulty swallowing Drugs that cannot be crushed Foods that can be mixed with drugs
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Special Populations – GERIATRIC PATIENTS (cont.)
Hearing loss Do they understand instructions? Have them repeat information back to
you
Difficulty reading small print Labeling Tablet colors
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Special Populations – GERIATRIC PATIENTS (cont.)
Short-term memory loss Written instructions Medication calendars Pill dispensers
Avoid OTC or herbal meds until discussing with physician.
Do not take expired meds or borrowed meds.
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Teaching Patients About Medications
1. Name of the medication
2. Purpose
3. How to store it
4. How long to take the medication
5. How and when to take it
6. How to know if it is effective
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Teaching Patients About Medications (cont.)
7. Required follow-up tests, doctor appointments
8. Possible side effects and what to do
9. Interactions with other drugs and foods
10. Symptoms to report to the doctor
11. What to do if a dose is missed
12. Keeping a list of all medications
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Dosages Based on Body Weight
Amount of medication per
Weight of the patient per
Unit of time
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Dosages Based on Body Weight
Rule 11-1Rule 11-1 Calculating dosage based on body weight:
1. Convert the patient’s weight to kilograms
2. Calculate the desired dose D by multiplying dose ordered by the weight in kilograms such as
dose desired kg or dose desired kg xkg
mcgx
kg
mg
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Dosages Based on Body Weight (cont.)
Rule 11-1Rule 11-1 (cont.)
3. Confirm whether or not the desired dose is safe by checking the label, package insert, or product literature.
4. Calculate the amount to administer, using fraction proportion, ratio proportion, dimensional analysis, or the formula method.
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Dosages Based on Body Weight (cont.)
Calculate the amount to administerto a 3-year-old weighing 34 lb.
Ordered: hysocyamine sulfate 5 mcg/kg subq 1 h pre-anesthesiaOn hand: hysocyamine sulfate 0.5 mg/mL
Convert 34 lb to kg = 15.5 kg
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Example Example
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Dosages Based on Body Weight (cont.)
Find the desired dose:
77.5 mcg = D
Find the amount to administer:77.5 mcg = 0.0775 = 0.08 mg
Amount to administer =
0.16 mL
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Example (cont.) Example (cont.)
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Dosages Based on Body Weight (cont.)
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Pediatric InjectionsStage of Development Maximum Volumes for
IM Injections
Infant 0.5 – 1 mL
Toddler, walking for at least 1 year
1 mL
Preschooler/elementary school age
1 – 1.5 mL
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Ensuring Safe Dosages
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Ensuring Safe Dosages (cont.)
Rule 11-2Rule 11-2 Ensuring Safe Dosages
When working with special populations, always check the package insert, drug label, or product literature to ensure the safety of the dose to be administered.
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Error Alert !
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Error Alert !Weight 8 lb 6 oz Convert 6 ounces to pounds
using as the conversion
Therefore 8 lb 6 oz = 8.375 lb
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Practice
Determine whether the following order is safe. If safe, calculate the amount to administer.
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Patient: Child who weighs 14.5 kgOrdered: Amoxil 75 mg PO q8hOn hand: Usual child dose 20-40 mg/kg day q8h
Answer Dosage doesn’t fall within recommended dosage range; contact the physician.
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Dosages Based on Body Surface Area (BSA)
BSA calculations
Provide more accurate dosage calculations that are specific to the patient’s size and severity of his/her illness.
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Dosages Based on BSA – CALCULATING BSA
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Dosages Based on BSA – CALCULATING BSA (cont.)
Rule 11-3 Rule 11-3 Calculating the BSA using a formula:
1. If you know the height in cm and weight in kg, calculate
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Dosages Based on BSA – CALCULATING BSA (cont.)
Rule 11-3 Rule 11-3 (cont.)
2. If you know the height in inches and weight in pounds, calculate
If the result is less then one, round to nearest hundredth.
If the result is greater than one, round to nearest tenth.
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Dosages Based on BSA – CALCULATING BSA (cont.)
Find the body surface area for an adult who is 5’6” talland who weighs 168 lb.
BSA = 1.88 m2
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Example Example
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Dosages Based on BSA – CALCULATING BSA (cont.)
Rule 11-4 Rule 11-4 Calculating BSA using a Nomogram:
Using a straight edge, align the straight edge so it intersects at the height and weight.
Doing so will create an intersection in the BSA scale.
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Dosages Based on BSA – CALCULATING BSA (cont.)
Find the body surface for a baby who is 24 in and weighs 14 lb and 8 oz.
Use the “Child’s Nomogram”
BSA = 0.21 m2
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Example Example
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Calculating Dosage Based on BSA
Rule 11-5 Rule 11-5 Calculating dosage based on BSA:1. Calculate the patient’s BSA.
2. Calculate the desired dose:
dosage ordered per m2 x BSA = desired dose
3. Confirm whether the desired dose is safe.
4. Calculate the amount to administer, using fraction proportion, ratio proportion, or the formula method.
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Calculating Dosage Based on BSA (cont.)
Ordered: CeeNU (1st dose) 140 mg now for a child 38 in tall and
weighing 47 lbAccording to the package the first dose is a single
oral dose providing 130 mg/m2 .
BSA = 0.76 m2
Desired dose should be 98.8 mg (based on recommended dosing)
Ordered dose of 140 mg is too large for the patient.
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Example Example
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PracticePatient: 34 cm tall, 5 kgOrdered: Cerubidine 5.5 mg IV weeklyOn hand: Cerubidine injection 5mg/mL when
reconstituted Recommended pediatric dose 25 mg/m2
If dose is safe, calculate amount to administer:
BSA = 0.22 m2 Desired dose = 5.5 mg
Dose ordered is a safe dose
Amount to administer = 1.1 mL
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Daily Maintenance Fluid Needs (DMFN)
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Daily Maintenance Fluid Needs (DMFN)(cont.)
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Daily Maintenance Fluid Needs (DMFN) (cont.)
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Daily Maintenance Fluid Needs (DMFN) (cont.)
Amount of maintenance fluid required varies by weight
Replacement fluids Based on patient’s condition
VomitingDiarrheaFever
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Daily Maintenance Fluid Needs (DMFN) (cont.)
Rule 11-6 Rule 11-6 To calculate daily maintenance fluid needs (DMFN) based on weight:
1. If the patient weighs up to 10 kg, find
2. If the patient weighs 10 to 20 kg, find
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Daily Maintenance Fluid Needs (DMFN) (cont.)
Rule 11-6Rule 11-6(cont.)
3. If the patient weighs over 20 kg, find
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Daily Maintenance Fluid Needs (DMFN) (cont.)
Find the DMFN for
a patient weighing 16 kg.
DMFN mL =
DMFN mL = 1000 + 300
DMFN mL = 1300 mL
Find the DMFN for a patient weighing 24 kg.
DMFN mL =
DMFN mL = 1500 + 80
DMFN mL = 1580 mL
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Examples Examples
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Daily Maintenance Fluid Needs (DMFN) (cont.)
Rule 11-7Rule 11-7For pediatric patients and critically ill patients, the amount of solution in the IV tubing must be considered when determining infusion times and volumes.
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Daily Maintenance Fluid Needs (DMFN) (cont.)
Standard IV tubing contains 10 mL of solution per five feet. When using a volume control chamber
and standard tubing, medication will not reach patient until this 10 mL infuses.
Low-volume tubing contains only 0.3 mL of solution per five feet.
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Practice Mrs. Robin had an oral intake of 750 mL. She
weighs 145 lb How much more is required for the patient’s DMFN?
Convert wt to kg: 65.9 kg = 66 kg
DMFN = = 2420mL
2420 mL – 750 mL = 1670 mL
She requires 1670 mL to meet her DMFN.
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Creatinine Clearance
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Creatinine Clearance (cont.)
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Creatinine Clearance (cont.)
Diseases that can damage kidneys Hypertension Diabetes Congestive heart failure
Drugs that alter or change kidney function Lasix Aminoglycoside antibiotics
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Creatinine Clearance (cont.)
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Creatinine Clearance (cont.)
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Ideal and Actual Body Weight Geriatric patients – decreased proportion
of lean body mass and water Alters distribution of drugs Monitor serum drug levels
Some water-soluble drugs strongly bound to lean tissue
Fat-soluble drugs distributed to body fat Slower release into circulation Residual effects
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Ideal and Actual Body Weight (cont.)
Water soluble drug doses for Overweight patients – based on ideal
body weight
Underweight patients (below ideal body weight) – based on actual weight
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Ideal and Actual Body Weight (cont.)
Rule 11-8 Rule 11-8 Determining safe dosages for geriatric patientsCheck the package insert or product literature and check if dose ordered is safe based on renal function and ideal or actual patient weight.
If the dose is safe, calculate the amount to administer.
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Determine Safe Dosages for Geriatric Patients
A 78-year-old male is 5’4” tall and weighs 180 lb. (Ideal weight range is
122–157 lb.) He has normal renal function and has a nonlife-threatening infection.
Ordered: Garamycin 85 mg IM q 8hOn hand: Garamycin injection, 40 mg/mL (usual
dosage is 1 mg/kg)
Convert ideal body weight to kg: 55 to 71 kg
Safe dosage range = 55 mg to 71 mg
85 mg does not fall within this range.
Contact the physician.
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Example Example
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Error Alert!
For medications that are strongly bound to lean body tissue, calculate an overweight patient’s dose on ideal body weight, not actual weight.
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PracticeMr. Adams weights 172 lb (at ideal body weight). He
is 5 ft, 7 in tall. CLCR 60 mL/min
Ordered: Vancocin HCl 150 mg IV q6hDaily recommended dosage for patients with normal
renal function 2g in divided doses. Recommended daily dose for patients with creatinine clearance of 60 mL/min is 925 mg/24 h.
Is dose ordered safe?
Yes 600 mg/24 h, which is less than 925 mg/24 h
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Polypharmacy
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Drug Interactions
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Drug Interactions (cont.)
Rule 11-9Rule 11-9 To identify cases of polypharmacy and reduce the risk of drug interactions, ask elderly patients about:
1. All medications they take which are prescribed by either their primary physician or specialists
2. Any over-the-counter medications they take
3. Any social drugs which they use
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Drug Interactions (cont.)
Rule 11-9 Rule 11-9 (cont.)
4. Medications that they borrow from family and friends
5. Herbal and home remedies that they use
6. Bringing all medications they take to be checked.
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Drug Interactions (cont.)
Factors causing adverse drug reactions Advanced age
Small body size
Multiple illnesses
Multiple medications
Living alone
Malnutrition
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Drug Interactions (cont.)
Drugs to Avoid in Specific Diseases
Severe Risk Drugs
Benign prostatic hypertrophy
Antihistamines, anti-Parkinson’s drugs, GI antispasmodics, antidepressants
Cardiac dysrhythmia Tricyclic antidepressants
Clotting disorders Antiplatelet drugs, aspirin
COPD Hypnotics, sedatives, beta blockers
GI diseases NSAIDs, aspirin
Seizures Metoclopramide (Reglan)
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Drug Interactions (cont.)
Drugs to Avoid in Specific Diseases
Less Severe Risk Drugs
Benign prostatic hypertrophy
Narcotics
ConstipationAntihistamines, anti-Parkinson’s drugs, GI antispasmodics, antidepressants
Diabetes mellitus Steroids, beta blockers
GI diseases Aspirin, potassium supplements
Insomnia Decongestants, bronchodilators, some antidepressants
Seizures Antipsychotics
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Apply Your Knowledge
True or FalseA baby who weighs 6 lb 8 oz weighs
6.8 lb.
A baby who weighs 9.5 lb weighs 9 lb 8 oz.
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Answer False
Answer True
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Apply Your Knowledge
What are three resources you can consult to see if the calculated dose is a safe dose to administer?
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Answer 1. Drug label2. Package insert3. Drug literature
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Apply Your Knowledge
Pediatric dosages should be rounded to the nearest: a. Tenthb. Hundredthc. Thousandth
What two things are pediatric dosages often based on?
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Answer Weight and BSA
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Apply Your Knowledge
Answer
Daily maintenance fluid needs (DMFN)
What is the amount of fluid the patient needs over a 24-hour period called?
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Apply Your Knowledge
Which age-related change would indicate a need to consider a smaller needle length for an IM injection?a. thinner, more fragile skinb. loss of subcutaneous tissuec. decreased muscle massd. difficulty swallowing
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Apply Your Knowledge
The test used to measure the rate at which the kidneys filter the blood is called:a. kidney function test creatinineb. renal function testc. creatinined. creatinine clearance
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Apply Your Knowledge
True or FalseFor medications that are strongly bound to lean body tissue, calculate an overweight patient’s dose on the ideal weight, not actual weight.
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Answer True
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Apply Your Knowledge
Polypharmacy occurs when a patient:
a. uses too many pharmacists
b. takes many medications at a time
c. borrows a medication from a friend
d. sees only one physician
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End of Chapter 11
Perfection consists not in doing extraordinary things, but in doing ordinary things extraordinarily well.
~Angelique Arnauld
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