Factors Influencing Drug Action and Pharmacokinetics (with Lactation Notes)

I. Why Is This Important?

The same medicinal product can have different effects in different people, or even in the same patient under different conditions.
This depends on multiple factors that influence:

• Pharmacokinetics (PK): “What does the body do to the drug?” (absorption, distribution, metabolism, elimination).
• Pharmacodynamics (PD): “What does the drug do to the body?” (interaction with the target, intensity of the effect).

In breastfeeding women, these factors determine not only the maternal effect and toxicity, but also drug transfer into breast milk and potential exposure of the breastfed infant.

II. Major Groups of Factors (with Examples)

1. Physiological Factors

Age

• Neonates and children

PK: Immature hepatic function (↓ CYP450), underdeveloped kidneys (↓ GFR), a higher percentage of total body water.
PD: A developing nervous system, often more sensitive to certain medicines.

Example: Chloramphenicol in neonates → “grey baby syndrome” (immature UDP‑glucuronyl transferase cannot conjugate the drug → toxicity).

From a lactation standpoint, drugs with known neonatal toxicity (e.g. chloramphenicol) require extreme caution or avoidance in breastfeeding mothers of premature or medically fragile infants, because any transferred amount may have a disproportionate impact on the immature neonatal organs.

• Older adults (>65 years)

PK: Reduced liver mass and blood flow (↓ metabolism), reduced renal function (↓ elimination), lower percentage of body water and higher percentage of adipose tissue.
PD: Changes in receptor number and sensitivity.

Example: Diazepam (lipophilic) → prolonged half-life in older adults (tissue accumulation + ↓ metabolism) → risk of oversedation and falls.

In breastfeeding women approaching older age, prolonged half‑life increases both maternal adverse effects and the duration of drug presence in plasma, which can increase the cumulative transfer into milk and the risk of infant sedation.

Sex (Biological Sex Differences)

PK: Women may have lower hepatic metabolic capacity for some CYP enzymes and a smaller volume of distribution for hydrophilic drugs.
PD: Hormonal differences can modulate drug sensitivity.

Example: Zopiclone (hypnotic) → women may show higher plasma concentrations and stronger clinical effect.

In lactating women, sex‑specific PK (e.g. slower clearance, higher systemic exposure) can translate into higher milk concentrations and more pronounced pharmacodynamic effects in the infant, especially for CNS depressants and beta‑blockers.

Genetics (Pharmacogenetics)

Polymorphisms in genes encoding metabolic enzymes or drug targets (receptors, enzymes).

• Example 1 (PK): Codeine → activated to morphine by CYP2D6.
• “Ultra‑rapid metabolizers” (high CYP2D6 activity) → high risk of morphine toxicity (respiratory depression).
• “Poor metabolizers” (low CYP2D6 activity) → weak analgesic effect.

In breastfeeding, a mother who is an ultra‑rapid CYP2D6 metabolizer may produce high morphine levels after standard codeine doses, leading to serious or even fatal toxicity in the breastfed infant; therefore, codeine is generally discouraged or contraindicated in lactation in many guidelines.

• Example 2 (PD): Warfarin (anticoagulant) → its effect is strongly influenced by polymorphisms in VKORC1 (target) and CYP2C9 (metabolism).
  In lactating mothers, careful dosing and monitoring remain essential to avoid maternal bleeding; warfarin transfer into milk is minimal, but maternal over‑anticoagulation still poses indirect risks.

2. Pathological Conditions

Hepatic impairment (Hepatitis, Cirrhosis)

PK: ↓ synthesis of plasma proteins (↑ free fraction of highly protein‑bound drugs), ↓ metabolism (CYP450), ↓ bile flow.

Example: Lorazepam (metabolized mainly by glucuronidation) → markedly prolonged half‑life in cirrhosis → risk of coma.

In lactating women with liver disease, reduced metabolism can markedly increase systemic exposure and drug transfer into milk; sedatives such as benzodiazepines may lead to infant hypotonia and poor suckling.

Renal impairment

PK: ↓ glomerular filtration rate (GFR) → accumulation of drugs primarily eliminated renally.

Example: Gentamicin (aminoglycoside) → accumulation → nephrotoxicity and ototoxicity; dose adjustment is mandatory.

In breastfeeding mothers with renal failure, reduced clearance prolongs exposure and may raise milk concentrations, although aminoglycosides are poorly absorbed orally by infants. Still, high maternal toxicity risk and possible disruption of breastfeeding must be considered.

Heart failure

PK: ↓ perfusion of liver and kidneys → ↓ metabolism and elimination; presence of oedema → altered distribution.

Example: Lidocaine → lower hepatic metabolism → risk of toxicity (seizures).

In lactating women with heart failure, PK alterations can intensify the effect of antiarrhythmics, beta‑blockers and diuretics, increasing both maternal adverse effects and the potential for clinically relevant exposure of the infant via milk.

3. Drug-Related Factors

Pharmaceutical form and route of administration

Bioavailability may vary substantially (e.g. modified‑release oral forms).

Example: Nitroglycerin → sublingual administration → rapid effect within minutes; topical ointment → slow, prolonged effect.

For lactating mothers, routes that bypass first‑pass metabolism (sublingual, transdermal, IV) often produce higher and sharper peaks in plasma concentration, which may transiently increase milk levels and infant exposure, especially for lipophilic drugs.

Chemical properties

Lipophilicity/hydrophilicity, pKa, degree of plasma protein binding.

These parameters also determine the degree of passage into breast milk: lipophilic, weakly basic, poorly protein‑bound drugs with low molecular weight and high maternal plasma concentration tend to accumulate more in milk and may require specific lactation risk assessment.

4. External Factors (Environment, Habits)

Drug interactions (Critical!)

• PK interactions: Modulation of absorption, metabolism (CYP induction/inhibition), and excretion.

Example (PK): Carbamazepine (potent CYP3A4 inducer) → markedly ↓ plasma concentration of simvastatin (CYP3A4 substrate) → statin inefficacy → increased risk of myocardial infarction.

In lactating women, enzyme induction (e.g. by carbamazepine, rifampicin) can reduce the effectiveness of co‑medications (including some antihypertensives or antiepileptics), while enzyme inhibition (e.g. by macrolides, azoles, grapefruit juice) can increase both maternal and infant exposure to critical drugs.

• PD interactions: Effects at the level of targets or overall pharmacodynamic response (synergism, antagonism).

Example (PD): Warfarin + aspirin (both “thin the blood” via different mechanisms) → synergism → high bleeding risk.

In lactation, additive PD effects (e.g. combined CNS depressants or combined antihypertensives) may also occur in infants when multiple drugs reach milk, even at low concentrations.

Food

Food can modify absorption (e.g. milk ↓ absorption of tetracycline) or metabolism (grapefruit juice is a strong CYP3A4 inhibitor).

For breastfeeding mothers, the timing of drug administration relative to meals and nursing can be used strategically (e.g. taking a drug immediately after breastfeeding to allow partial elimination before the next feed), especially when food affects rate and extent of absorption.

Smoking (Tobacco)

Induces CYP1A2 (and other enzymes) → ↑ metabolism of substrates.

Example: Theophylline → faster metabolism in smokers → weaker effect; when a smoker stops, theophylline concentration ↑ → toxicity risk.

Smoking during lactation not only alters drug kinetics but also adds direct toxic exposure (nicotine, combustion products) to the infant through both inhalation and breast milk, and can modify the effect of co‑administered drugs (e.g. theophylline, clozapine).

Alcohol

Acute intake: Inhibits some CYP enzymes → ↑ concentration of certain medicines.
Chronic intake: Induces CYP2E1 (↓ concentration of some drugs) and damages liver and kidneys.

In breastfeeding women, acute alcohol use can transiently increase drug levels and alter milk composition, while chronic misuse leads to organ damage, impaired drug handling and significant risks for the infant’s neurodevelopment and environment of care.

Key Messages (Macro-Level Knowledge)

• Drug action is not constant; it depends on multiple interacting factors.
• Physiology: Age, sex, and genetic background are fundamental; CYP polymorphisms are frequent and clinically important.
• Pathology: Hepatic and renal impairment dramatically change PK and often PD; organ function must always be assessed.
• Interactions are especially common and dangerous:
• Inducers (e.g. carbamazepine, rifampicin) → ↓ concentrations → loss of efficacy.
• Inhibitors (e.g. clarithromycin, grapefruit juice) → ↑ concentrations → toxicity.
• PD interactions (agonism/antagonism) → potentiation or attenuation of effects.

From a lactology perspective, these same principles govern maternal exposure, milk transfer and infant dose: high‑risk combinations, organ dysfunction and genetic extremes (e.g. rapid metabolizers) can convert seemingly “compatible” drugs into clinically relevant hazards for the breastfed infant.

Clinical Relevance

• Individualization of therapy: Doses must be adjusted according to age, body weight, organ function, genetic factors and concomitant medication.
• Monitoring: For drugs with a narrow therapeutic index (warfarin, digoxin, antiepileptics), it is vital to follow plasma concentrations or biological markers (e.g. INR for warfarin).
• Prevention: Always check for potential drug interactions before prescribing a new medicine!

In breastfeeding, individualization should routinely include lactation status, age and condition of the infant, timing of feeds, and the physicochemical profile of the drug. Close monitoring of both mother and child, and proactive prevention of high‑risk interactions, are central to safe pharmacotherapy during lactation.