The same dose of the same drug can produce very different effects in different people. Body composition, organ maturity or decline, pregnancy physiology, genetics, and disease change pharmacokinetics (how much drug reaches the target) and pharmacodynamics (how sensitive the target is). Four groups need particular care: pregnant clients, breastfeeding clients and their infants, children, and older adults — along with anyone with kidney or liver impairment.
How physiology changes drug handling
| Factor | Pregnancy | Neonates and infants | Older adults |
|---|
| Absorption | Slower gastric emptying; nausea and vomiting | Variable gastric pH and emptying; thin skin absorbs topical drugs readily | Slower gastric emptying (usually minor effect) |
| Body water and fat | ↑ plasma volume and body water | High total body water → larger volume of distribution for water-soluble drugs (higher mg/kg doses of some drugs) | ↓ body water → higher levels of water-soluble drugs (lithium, digoxin, alcohol); ↑ body fat → fat-soluble drugs last longer (diazepam) |
| Protein binding | ↓ albumin → more free drug | ↓ albumin; bilirubin can be displaced by some drugs (sulfonamides, ceftriaxone) → kernicterus risk | ↓ albumin (illness, malnutrition) → more free drug |
| Metabolism | Some enzymes increase (e.g., lamotrigine clearance rises) | Immature liver enzymes (glucuronidation) in newborns | ↓ liver blood flow and mass → slower clearance of some drugs |
| Excretion | GFR rises about 50% → faster clearance of renally excreted drugs (lithium, some antibiotics) | Immature kidneys in newborns; function matures over the first 1–2 years | GFR declines with age; serum creatinine may look normal because of low muscle mass |
| Sensitivity | Drug reaches the fetus through the placenta | Immature blood–brain barrier; unique toxicities | ↑ CNS sensitivity (sedation, delirium), impaired baroreceptor reflex (orthostatic hypotension) |
Individual (genetic) differences also matter: enzyme variants (CYP2D6, CYP2C19), HLA types that predict severe skin reactions, and G6PD deficiency (see Topics 1 and 4).
Pregnancy — timing of exposure
- First 2 weeks after conception: "all-or-none" period — the embryo either does not survive or develops normally
- Organogenesis (about weeks 3–8 after conception, or about weeks 5–10 of gestation counted from the last menstrual period): highest risk for structural birth defects
- Fetal period: risk of growth restriction, organ dysfunction, and neonatal effects (withdrawal, respiratory depression, bleeding)
Pregnancy labeling. In the US, the Pregnancy and Lactation Labeling Rule (effective 2015) replaced the letter categories A, B, C, D, and X with narrative sections: 8.1 Pregnancy (risk summary, clinical considerations, data), 8.2 Lactation, and 8.3 Females and Males of Reproductive Potential (pregnancy testing, contraception, infertility). Older references and some countries still quote letter categories; learn the drugs and risks rather than the letters.
Known teratogens and fetotoxic drugs
| Drug or class | Risk | Key nursing point |
|---|
| Isotretinoin | Severe malformations | Strict pregnancy prevention program (two forms of contraception, repeated pregnancy tests) |
| Valproate | Neural tube defects, lower IQ, autism risk | Avoid in people who can become pregnant unless no alternative; folic acid |
| Carbamazepine, phenytoin, phenobarbital, topiramate | Malformations | Do not stop antiseizure drugs abruptly — seizures also harm the fetus; plan pregnancy with the prescriber |
| Warfarin | Embryopathy, fetal bleeding | Usually switched to low molecular weight heparin; exceptions in some mechanical valves |
| ACE inhibitors, ARBs, direct renin inhibitors | Fetal kidney failure, oligohydramnios, skull hypoplasia (second and third trimester) | Stop as soon as pregnancy is detected (boxed warning) |
| Methotrexate, misoprostol, thalidomide, mycophenolate | Pregnancy loss, malformations | Contraindicated in pregnancy (non-cancer uses); pregnancy prevention programs |
| Tetracyclines | Tooth discoloration, bone effects (from second trimester) | Avoid in pregnancy |
| Aminoglycosides (streptomycin) | Fetal hearing loss | Avoid when alternatives exist |
| NSAIDs | Oligohydramnios and fetal kidney dysfunction from 20 weeks; premature ductus arteriosus closure in the third trimester | Avoid from 20 weeks unless specifically directed (low-dose aspirin for preeclampsia prevention is an exception) |
| Statins | Formal contraindication removed (FDA 2021); most clients stop when pregnancy is confirmed | Individualized decision for very high-risk clients |
| Lithium | Small increase in cardiac malformation (Ebstein anomaly) risk; neonatal toxicity | Benefit may outweigh risk; monitor levels closely as clearance changes |
Commonly preferred in pregnancy (under provider direction): acetaminophen/paracetamol for pain and fever at the lowest effective dose for the shortest time; insulin (and some oral agents) for diabetes; labetalol, nifedipine, methyldopa for hypertension; low molecular weight heparin for anticoagulation; penicillins and cephalosporins for many infections; folic acid supplementation.
Lactation. Most drugs are compatible with breastfeeding. Transfer into milk is higher with drugs that are small, lipid-soluble, poorly protein-bound, long half-life, and taken at high doses. Risk is highest for preterm and newborn infants.
- Avoid or use special caution: codeine and tramadol (fatal infant toxicity in ultrarapid metabolizers — not recommended during breastfeeding), most chemotherapy, radioactive drugs, amiodarone, lithium (infant monitoring), high-dose or long-term benzodiazepines (sedation)
- Strategies: choose drugs with established safety data; take the dose just after a feeding; watch the infant for sedation, poor feeding, rash, or diarrhea
- Evidence-based references (e.g., the National Library of Medicine's LactMed database) guide choices; stopping breastfeeding is rarely necessary
Children
- Doses are weight-based (mg/kg) or occasionally based on body surface area (chemotherapy); a child's dose should not exceed the adult dose; verify every pediatric dose against a pediatric reference
- Example: acetaminophen/paracetamol 10–15 mg/kg per dose every 4–6 hours, not more than 75 mg/kg/day, 1 g per dose, or 4 g/day — whichever is less, and no more than 5 doses in 24 hours; ibuprofen generally not used under 6 months
| Drug | Pediatric concern |
|---|
| Aspirin | Avoid in children and teenagers with viral illness (Reye syndrome) |
| Codeine, tramadol | Contraindicated under 12 years and after tonsillectomy/adenoidectomy under 18 years — fatal respiratory depression |
| Tetracyclines | Avoid under 8 years (permanent tooth staining); short doxycycline courses may be used for specific serious infections |
| Fluoroquinolones | Tendon and joint effects — reserved for specific indications |
| Ceftriaxone | Avoid in hyperbilirubinemic neonates; never give with IV calcium-containing solutions in neonates (fatal precipitates) |
| Chloramphenicol | Gray baby syndrome in newborns (immature glucuronidation) |
| Promethazine | Contraindicated under 2 years (fatal respiratory depression) |
| Over-the-counter cough and cold products | Not for children under 2 years (FDA); labeled not for use under 4 years |
Older adults
- "Start low, go slow"; review every drug for continued need (deprescribing)
- Polypharmacy (commonly defined as 5 or more drugs) raises the risk of interactions, falls, delirium, and hospital admission
- Prescribing cascade: an adverse effect is treated as a new condition with another drug (e.g., ankle edema from amlodipine treated with a diuretic)
- Estimate kidney function with creatinine clearance or eGFR, not creatinine alone
Beers Criteria (American Geriatrics Society, 2023) — examples of drugs to avoid or use with caution
| Drug or class | Why |
|---|
| First-generation antihistamines (diphenhydramine, hydroxyzine, chlorpheniramine) | Anticholinergic: confusion, constipation, urinary retention, falls |
| Benzodiazepines, Z-drugs (zolpidem) | Delirium, falls, fractures, cognitive impairment |
| Tricyclic antidepressants (amitriptyline), muscle relaxants | Anticholinergic, sedation, orthostatic hypotension |
| Sulfonylureas — avoid as first- or second-line therapy; if needed, short-acting glipizide rather than glyburide or glimepiride | Prolonged hypoglycemia, cardiovascular risk |
| Sliding-scale insulin alone | Hypoglycemia without better control |
| Long-term NSAIDs | GI bleeding, kidney injury, heart failure |
| Antipsychotics for behavioral symptoms of dementia | Increased mortality and stroke (boxed warning) |
| PPIs beyond 8 weeks without a clear indication | C. difficile, bone loss, fractures |
| Aspirin for primary prevention | Bleeding risk outweighs benefit |
| Warfarin as initial therapy for atrial fibrillation or VTE | A DOAC is preferred unless contraindicated |
- Pregnancy: congenital anomalies, fetal growth restriction, neonatal withdrawal (opioids, SSRIs late in pregnancy — transient adaptation symptoms), neonatal respiratory depression after opioids in labor, neonatal bleeding (warfarin)
- Breastfed infants: sedation, poor feeding, respiratory depression (opioids), diarrhea or thrush (antibiotics)
- Children: dosing errors (mg vs mL, decimal errors, adult concentration given to a child), paradoxical excitement with antihistamines and benzodiazepines, dehydration-related toxicity
- Older adults: delirium, falls, orthostatic hypotension, constipation, urinary retention, hypoglycemia, bleeding, hyponatremia (SSRIs, thiazides) — often presenting atypically as confusion or functional decline
- Confirm pregnancy status before starting known teratogens; use the labeled contraception and testing requirements
- Do not stop drugs for chronic conditions (epilepsy, depression, asthma, hypertension, HIV) on discovering pregnancy without consulting the prescriber — untreated disease also harms mother and fetus
- Kidney impairment: reduce dose or extend the interval for renally cleared drugs (aminoglycosides, vancomycin, enoxaparin, digoxin, lithium, gabapentin); metformin contraindicated with eGFR below 30 mL/min/1.73 m²
- Liver impairment: caution with opioids, benzodiazepines, acetaminophen (lower maximum), and drugs with high first-pass metabolism
- Children: use only pediatric formulations and concentrations; avoid adult combination products
- Older adults: avoid combining multiple CNS depressants or anticholinergics; watch for interactions with warfarin and digoxin
Listed in priority order.
- Safety screening before every new drug: weight in kilograms (children), pregnancy and breastfeeding status, age, kidney and liver function, allergies, and the full medication list including over-the-counter and herbal products
- Pediatric dose safety: calculate the mg/kg dose and the safe range; compare with the ordered dose; independent double check where policy requires; use oral syringes and pediatric concentrations
- Older adults: fall precautions with sedating or blood pressure drugs; orthostatic vital signs; assess cognition for new delirium; monitor intake, output, and electrolytes with diuretics
- Pregnancy and lactation: confirm the drug is appropriate; teach risks and benefits; observe breastfed infants when the parent takes sedating drugs
- Monitor drug levels and organ function for narrow-range drugs; pregnancy can lower levels (lithium, lamotrigine) and aging can raise them
- Medication review and deprescribing: report drugs that may be causing new symptoms (possible prescribing cascade)
- Adherence support: pill organizers, simplified schedules, large-print labels, caregiver involvement
- Tell every provider if you are pregnant, planning pregnancy, or breastfeeding; do not start or stop drugs without advice
- Take folic acid before and during early pregnancy as advised
- Use acetaminophen/paracetamol rather than NSAIDs for pain in later pregnancy unless told otherwise, at the lowest effective dose
- Parents: dose by the child's weight; measure in mL with an oral syringe, never a kitchen spoon; do not give aspirin to children; check combination products for duplicate acetaminophen
- Keep all drugs locked and out of children's reach
- Older adults and caregivers: bring all bottles (including vitamins and herbal products) to each visit; rise slowly; report confusion, falls, or dizziness; avoid over-the-counter sleep aids containing diphenhydramine
- Pediatric overdose: most often from accidental ingestion or a 10-fold dosing error; acetaminophen (antidote acetylcysteine), iron (deferoxamine), opioids and fentanyl patches (naloxone), clonidine, and adult sulfonylureas (prolonged hypoglycemia — glucose, octreotide) are especially dangerous
- Older adults: toxicity often presents as delirium, falls, bradycardia, or hypoglycemia rather than classic signs; digoxin, lithium, and anticholinergic toxicity are common
- Neonatal opioid withdrawal after in-utero exposure: irritability, high-pitched cry, poor feeding, tremors — non-drug care first (swaddling, rooming-in, breastfeeding when appropriate), then drug treatment if needed
- Magnesium sulfate in pregnancy: toxicity (loss of reflexes, respiratory depression) is reversed with calcium gluconate
- Organogenesis (weeks 3–8 after conception = about 5–10 weeks' gestation) is the highest-risk period for structural defects
- US labeling replaced pregnancy letter categories (PLLR 2015) with narrative Pregnancy, Lactation, and Reproductive Potential sections
- Major teratogens: isotretinoin, valproate, warfarin, ACE inhibitors/ARBs, methotrexate, misoprostol, thalidomide, tetracyclines
- Avoid NSAIDs from 20 weeks of pregnancy
- Breastfeeding: most drugs compatible; avoid codeine and tramadol; dose right after a feeding
- Pediatric doses are mg/kg, never above the adult dose; double check
- No aspirin in children with viral illness (Reye); no codeine or tramadol under 12; no promethazine under 2
- Ceftriaxone + IV calcium in neonates — contraindicated
- Older adults: ↓ body water, ↑ fat, ↓ GFR — "start low, go slow"
- Beers Criteria: avoid first-generation antihistamines, benzodiazepines, Z-drugs, and sulfonylureas as first- or second-line therapy in older adults
- Normal serum creatinine does not mean normal kidney function in older adults
Country Notes
United States
- Pregnancy exposure registries are listed in drug labels under section 8.1. Isotretinoin is dispensed only through the iPLEDGE REMS program.
- In September 2025 the FDA started a label change about a possible association between prenatal acetaminophen use and neurodevelopmental disorders, while stating that a causal relationship has not been established; ACOG continues to recommend acetaminophen as the analgesic and antipyretic of choice in pregnancy, used judiciously. As of mid-2026 the product label had not yet been revised — check the current label.
Philippines
- Paracetamol is the standard name on labels and prescriptions; pediatric drops and syrups come in different concentrations, so the concentration must be checked on every product.
- Letter pregnancy categories still appear in some older references and product materials; teach the specific drug risks rather than relying on letters.