Identification of medication related problems

Identification of Medication

Medication-Related Problems (MRPs)

Medication-related problem (MRP):
An event or circumstance involving drug therapy that actually or potentially interferes with optimal patient outcomes.

Classification of Medication-Related Problems

1.     Untreated Condition

• Patient has a medical condition requiring therapy but no drug is prescribed.

2.     Drug Use Without Indication

• Drug taken without a valid medical reason.

3.     Improper Drug Selection

• Wrong drug chosen for the condition
• Drug unsuitable for patient-specific factors (age, pregnancy, renal/hepatic disease)

4.     Sub-therapeutic Dosage

• Correct drug given but dose is too low to achieve therapeutic effect.

5.     Overdosage

• Correct drug given but dose is too high, causing toxicity.

6.     Adverse Drug Reactions (ADRs)

• Harmful, unintended response at normal doses.
• Common in elderly → worsens geriatric problems:
• Urinary incontinence
• Constipation
• Weight loss

7.     Drug Interactions

• Negative interaction between:
• Drug–drug
• Drug–food
• Drug–chemical

8.     Failure to Receive Medication

• Patient does not receive medication due to:
• Economic barriers
• Psychological or social issues
• Pharmaceutical or distribution problems

2.1.2.1 Drug Interactions / Incompatibility

Drug interaction:
Occurs when the pharmacological effect of a drug is altered by another drug, food, drink, or environmental chemical.

• Object drug: Drug whose effect is altered
• Precipitant drug: Drug causing the interaction

📌 Drug interactions are a special type of adverse reaction with altered efficacy or toxicity.

Example:

• Probenecid + Penicillin → increased penicillin levels (reduced renal excretion)

Types of Drug Interactions

 Pharmacokinetic Interactions

Affect ADME of the object drug.

(a) Interactions Affecting Absorption

• Very common after oral administration
• May alter:
• Rate of absorption
• Extent of absorption
• Drugs with incomplete absorption are more susceptible.

Examples:

• Chelation
• Changes in gastric pH
• Altered GI motility

(b) Interactions Affecting Distribution

• Due to competition for plasma protein binding
• Risk is high when displaced drug:
• Is >95% protein bound
• Has low volume of distribution
• Has narrow therapeutic index

Examples:

• Warfarin
• Phenytoin

(c) Interactions Affecting Metabolism

• Most common and clinically significant
• Occurs due to:
• Enzyme induction
• Enzyme inhibition
• Especially important for drugs undergoing first-pass hepatic metabolism

Examples:

• Enzyme inducers → ↓ drug levels
• Enzyme inhibitors → ↑ drug toxicity

(d) Interactions Affecting Excretion

• Occur due to changes in:
• Glomerular filtration rate (GFR)
• Renal blood flow
• Tubular secretion
• Urine pH

Important example:

• Thiazide diuretics + Lithium
• ↓ lithium excretion
• ↑ lithium toxicity
  (pharmacodynamic effect leading to pharmacokinetic change)

Pharmacokinetic Drug Interactions – Summary Tables

1. Absorption Interactions

Mechanism	Object Drug(s)	Precipitant	Effect
Complexation / Adsorption	Tetracycline, Penicillin	Antacids, food, mineral supplements (Al, Mg, Fe, Zn, Ca)	Poorly soluble complex → ↓ absorption
Complexation	Ciprofloxacin, Norfloxacin	Antacids (Al, Mg, Ca)	↓ absorption
Alteration of GI pH	Sulphonamides, Aspirin	Antacids	↑ dissolution
Alteration of GI pH	Ferrous sulphate	Sodium bicarbonate, Calcium carbonate	↓ dissolution

2. Metabolism Interactions

(a) Enzyme Induction

Object Drug(s)	Precipitant	Effect
Corticosteroids, Oral contraceptives, Phenytoin	Barbiturates	↓ plasma levels
Corticosteroids, Oral contraceptives	Phenytoin	↓ plasma levels

(b) Enzyme Inhibition / Related Interactions

Object Drug / Substance	Precipitant	Effect
Tyramine-rich food (cheese, liver)	MAO inhibitors (Phenelzine)	Hypertensive crisis
Folic acid	Phenytoin	↓ folic acid absorption

3. Alteration of Gut Motility

Object Drug(s)	Precipitant	Effect
Aspirin, Diazepam, Levodopa, Lithium	Metoclopramide	Rapid gastric emptying
Levodopa, Lithium	Anticholinergics	Delayed gastric emptying

4. Distribution Interactions (Protein Binding Displacement)

Object Drug	Precipitant	Effect
Warfarin	Phenylbutazone, Chloral hydrate	↑ clotting time
Phenytoin	Valproic acid	Phenytoin toxicity
Tolbutamide	Sulfonamides	↑ hypoglycemic effect

5. Excretion Interactions

(a) Changes in Active Tubular Secretion

Object Drug(s)	Precipitant	Effect
Penicillin, PAS, Dapsone, Nalidixic acid	Probenecid	↑ plasma level → toxicity
Ranitidine, Procainamide	Cimetidine	Risk of toxicity

(b) Changes in Urine pH

Object Drug(s)	Precipitant	Effect
Amphetamine, Tetracycline	Antacids, Thiazides	↑ passive reabsorption of basic drugs

(c) Changes in Renal Blood Flow

Object Drug	Precipitant	Effect
Lithium	NSAIDs	↓ renal clearance → lithium toxicity

Pharmacodynamic Interactions

Definition
Pharmacodynamic interactions occur when the effect or activity of a drug at its site of action is modified by another drug, without altering drug concentration.
These interactions may be direct or indirect.

A. Direct Pharmacodynamic Interactions

Drugs having similar or opposing pharmacological effects are used together.

1. Antagonism

• One drug opposes the action of another.
• Effect of one drug is reduced or nullified by the other.

Example:

• Acetylcholine ↓ heart rate
• Noradrenaline ↑ heart rate → Opposing effects on heart rate

2. Addition (Summation)

• Two drugs with similar actions are given together.
• Combined effect = sum of individual effects

Example:

• CNS depressants: sedatives + hypnotics
  → Increased CNS depression

3. Synergism

• One drug enhances the action of another drug.
• Combined effect is greater than additive.

Example:

• Alcohol enhances analgesic effect of aspirin

B. Indirect Pharmacodynamic Interactions

• Drugs have unrelated primary actions.
• One drug indirectly alters the effect of another drug.

Example:

• Salicylates inhibit platelet aggregation
• Warfarin causes anticoagulation
  → Increased risk of bleeding due to impaired haemostasis

Quick Comparison (Exam Tip)

Type	Action	Effect
Antagonism	Opposite	Effect reduced
Addition	Similar	Effect added
Synergism	Enhancing	Effect amplified
Indirect	Unrelated	Effect modified