# Drug Interaction Criteria

## Severity Classification

### Critical/Major Interactions
**Definition**: Combinations that should be avoided because risk outweighs benefit

**Characteristics**:
- Life-threatening or severe harm potential
- Well-documented in clinical literature
- Safe alternatives typically available

**Examples**:
- MAOIs + SSRIs → Serotonin syndrome
- QT-prolonging drugs + other QT-prolonging agents → Torsades de pointes
- Warfarin + high-dose aspirin → Major bleeding risk
- Potent CYP3A4 inhibitors + narrow therapeutic index drugs

**Action**: Avoid combination; consider alternative therapies

### Moderate Interactions
**Definition**: Combinations requiring therapy modification or increased monitoring

**Characteristics**:
- Potential for clinically significant effects
- Well-documented in clinical literature
- May be used with appropriate precautions

**Examples**:
- Warfarin + antibiotics → Enhanced anticoagulation
- ACE inhibitors + NSAIDs → Reduced renal function
- Certain diuretics + lithium → Lithium toxicity

**Action**: Consider alternatives; monitor closely if necessary

### Minor Interactions
**Definition**: Combinations with limited clinical significance

**Characteristics**:
- Minimal effects; usually manageable
- Limited clinical documentation
- Usually do not require intervention

**Examples**:
- Antacids + oral medications → Reduced absorption
- Certain antibiotics with food → Reduced absorption
- H2 blockers with pH-dependent drugs

**Action**: Provide patient education; routine monitoring

## Interaction Mechanisms

### Pharmacokinetic Interactions

#### CYP450 Enzyme Interactions
**Major Enzymes**:
- CYP1A2: Theophylline, clozapine, caffeine
- CYP2C9: Warfarin, phenytoin, NSAIDs
- CYP2C19: Omeprazole, clopidogrel, diazepam
- CYP2D6: Codeine, metoprolol, fluoxetine
- CYP3A4/5: Simvastatin, cyclosporine, midazolam

**Interaction Types**:
- **Inhibition**: Reduced metabolism → increased drug levels
- **Induction**: Increased metabolism → decreased drug levels

#### Transport Protein Interactions
- **P-glycoprotein (P-gp)**: Digoxin, dabigatran
- **OATP**: Statins, antihistamines

#### Protein Binding Displacement
- Warfarin + sulfonamides
- Phenytoin + valproic acid

### Pharmacodynamic Interactions

#### Additive Effects
**CNS Depression**:
- Opioids + benzodiazepines + alcohol
- Risk: Respiratory depression, sedation

**Anticholinergic Effects**:
- Multiple anticholinergic medications
- Risk: Confusion, constipation, urinary retention

**QT Prolongation**:
- Multiple QT-prolonging drugs
- Risk: Torsades de pointes, sudden death

**Serotonergic Effects**:
- SSRIs + SNRIs + tramadol + MAOIs
- Risk: Serotonin syndrome

#### Synergistic Effects
- Enhanced efficacy or toxicity
- Example: Trimethoprim + sulfamethoxazole

#### Antagonistic Effects
- Reduced therapeutic efficacy
- Example: Beta-blockers + beta-agonists

## Contraindication Types

### Absolute Contraindications
**Never use in these conditions**:

- Beta-blockers in severe asthma
- ACE inhibitors in bilateral renal artery stenosis
- Succinylcholine in malignant hyperthermia
- NSAIDs in active peptic ulcer bleeding

### Relative Contraindications
**Use with caution, benefit-risk assessment**:

- Beta-blockers in diabetes (may mask hypoglycemia)
- NSAIDs in heart failure (fluid retention)
- Anticholinergics in elderly (fall risk)

### Age-Based Contraindications
- Aspirin in children (Reye's syndrome)
- Tetracyclines in children <8 years (tooth discoloration)
- Certain antipsychotics in elderly dementia

### Organ Dysfunction Contraindications
- Metformin with severe renal impairment (lactic acidosis)
- Nitrofurantoin with renal failure (ineffective and toxic)
- Certain chemotherapeutics with low blood counts

## Allergy Cross-Reactivity

### Beta-Lactam Antibiotics
**Penicillin Allergy**:
- Cross-reactivity with cephalosporins: ~10%
- Cross-reactivity with carbapenems: ~1%
- Avoid: All penicillins

**Cephalosporin Allergy**:
- Partial cross-reactivity with penicillins
- Based on similar beta-lactam ring structure

### Sulfonamides
**Sulfa Antibiotic Allergy**:
- Cross-reactivity with non-antibiotic sulfonamides
- Loop diuretics (furosemide)
- Thiazide diuretics
- Sulfonylureas

**Note**: Cross-reactivity is due to sulfonamide moiety, not the entire structure

### NSAIDs
**Aspirin-Exacerbated Respiratory Disease (AERD)**:
- Cross-reactivity among most NSAIDs
- Safe: Acetaminophen, selective COX-2 inhibitors

### Local Anesthetics
**Ester-type**: Higher allergy potential
- Procaine, tetracaine

**Amide-type**: Lower cross-reactivity
- Lidocaine, bupivacaine

## Evidence Grading

### Established
- Consistent evidence from multiple high-quality studies
- Well-documented mechanism
- Clinical significance confirmed

### Probable
- Good evidence from controlled studies
- Strong clinical consensus
- Mechanism understood

### Possible
- Limited evidence or case reports
- Theoretical basis
- Requires further study

### Suspected
- Theoretical potential based on mechanism
- Limited clinical data
- Caution advised

## Monitoring Recommendations

### Laboratory Monitoring
- Drug levels (when applicable)
- Organ function tests
- Coagulation studies
- Electrolytes

### Clinical Monitoring
- Vital signs
- Symptom assessment
- Adverse effect screening
- Therapeutic response

### Timing
- Baseline (before starting)
- Early phase (3-7 days)
- Steady state
- Long-term monitoring