Prescribe safely for elderly patients, liver/renal impairment, pregnancy, lactation, children and women of childbearing potential.

Special Prescribing Requirements

Start from the simplest idea in all of prescribing: every standard adult dose is a prediction. When you write the usual amount of a drug, you are silently predicting that it will reach a useful concentration at its target, stay there long enough to work, then be cleared before it causes harm. That prediction rests on assumptions about how an average adult body absorbs, distributes, metabolises and eliminates a drug. Special prescribing is simply the discipline of noticing when those assumptions no longer hold, and adjusting before the patient is harmed.

In O&G the assumptions break constantly. The body in front of you may be a pregnant patient whose plasma volume has expanded and whose kidneys are filtering faster; a breastfeeding mother whose milk is a second route of delivery into a tiny infant; a patient with pre-eclampsia and falling urine output who can no longer clear a renally excreted drug; an older woman with low muscle mass, declining kidneys and eight other medicines; a teenager who needs confidential care; or a woman who could be pregnant before she knows it, with an embryo exposed in the most vulnerable weeks. In each case the "average adult" prediction is wrong, and prescribing safely means knowing which assumption has broken and why.

The safe prescribing chain follows that logic in order:

patient physiology -> indication -> drug exposure -> developmental or organ vulnerability -> interactions -> monitoring -> documentation -> review

This chapter builds from first principles. We start with the four pharmacokinetic levers (absorption, distribution, metabolism, elimination) that every special population alters in its own way, then add the placenta and breast as O&G-specific exposure routes, then work through each population, and finish with the evidence, regulatory and pharmacovigilance reasoning that makes the whole thing defensible. Detailed disease protocols and specific regimens belong in the linked clinical chapters; here we teach the mechanism that makes those protocols safe to apply.

The Four Levers: Why Exposure Changes

Before any population, anchor on what can actually change. A drug's effect depends on its concentration at the target over time, and that concentration is governed by four processes, abbreviated ADME:

• Absorption — how much drug enters the circulation, and how fast. Altered by gastric emptying, gut pH, gut blood flow, vomiting, and route of administration.
• Distribution — where the drug goes once absorbed. Governed by the volume of distribution (Vd), plasma protein binding (mainly albumin), body water, body fat and tissue perfusion.
• Metabolism — usually hepatic biotransformation. Phase I reactions (oxidation/reduction by cytochrome P450 enzymes) make a drug more polar; phase II reactions (conjugation, e.g. glucuronidation) attach a water-soluble group so the kidney can excrete it.
• Elimination — mostly renal excretion of the parent drug or its metabolites, governed by glomerular filtration rate (GFR) and tubular handling; some drugs leave via bile.

Two clinical quantities fall straight out of these levers and are worth holding onto for the rest of the chapter:

• The loading dose (the first, larger dose used to fill the body quickly) depends mainly on the volume of distribution and on how urgently a target concentration is needed.
• The maintenance dose (the ongoing dose) depends mainly on clearance — how fast the body removes the drug. A drug's half-life, the time for its concentration to halve, rises when clearance falls or when Vd expands.

Every special population in this chapter perturbs one or more of these levers. Pregnancy expands plasma volume and Vd, lowers albumin and speeds renal clearance. Renal impairment cuts elimination. Liver disease cuts metabolism and lowers albumin. The neonate has immature phase I and phase II metabolism and immature renal clearance. The frail older patient has reduced renal clearance and altered body composition. If you can name which lever has moved, the dosing logic follows; if you only memorise dose tables, you are lost the moment the patient is non-standard.

The Special-Population Question

Whenever a patient falls outside the usual adult assumptions, ask the same questions in the same order.

The exam trap is to name the population but not the mechanism. "Use caution in pregnancy" is too vague. A strong answer says whether the concern is teratogenicity, fetotoxicity, altered maternal clearance, uterine activity, neonatal adaptation, breastfeeding transfer, maternal disease risk or poor follow-up.

Women of Reproductive Potential

Women of reproductive potential are not a niche group in O&G; they are the default. Many pregnancies are unplanned, and the embryo may be exposed before the first missed period. Therefore, prescribing to any patient who could conceive requires pregnancy possibility, contraception and pregnancy intention to be considered without judgement.

The most important prescribing failure is preventable teratogen exposure. Valproate, retinoids, methotrexate, mycophenolate, warfarin in selected contexts, ACE-inhibitor or ARB exposure after early pregnancy, and some oncology or immunosuppressive drugs require more than a casual warning. The prescriber should document counselling, contraception, alternatives, pregnancy testing where indicated, and the plan if pregnancy occurs.