Pharmaceuticals are essential to modern healthcare but increasingly represent a pervasive and biologically active class of environmental contaminants. Following administration, many drugs are incompletely metabolised in the human body and are excreted as parent compounds or active metabolites, subsequently entering municipal wastewater treatment plants (WWTPs). Conventional treatment processes only partially remove many pharmaceuticals, resulting in chronic sub-therapeutic exposure of microbial communities which act as both functional agents of biodegradation and sensitive ecological targets. Such exposure alters microbial structure and function, reduces biotransformation capacity, promotes the persistence of recalcitrant compounds such as carbamazepine and diclofenac, and drives the selection and dissemination of antibiotic resistance genes. These effects may propagate across aquatic, terrestrial, agricultural, and food systems via treated effluents and biosolids, linking human medical practices to environmental and public health outcomes. By integrating Pharmaceutical science, wastewater engineering, microbiome ecology, and antimicrobial resistance research, this work frames pharmaceutical pollution as a closed-loop OneHealth challenge.
The continuous development of new pharmaceutical compounds is a fundamental requirement for sustaining and advancing global healthcare systems. Escalating challenges such as antimicrobial resistance, the increase in chronic and age-related diseases, and the emergence of novel pathogens necessitate the discovery of drugs with improved efficacy, selectivity, and safety profiles [
1,
2]. For instance, the declining clinical effectiveness of legacy antibiotics has prompted renewed efforts to identify compounds with novel mechanisms of action, while advances in oncology have driven the development of targeted small-molecule inhibitors and biologics that aim to minimise systemic toxicity [
3]. However, despite increasing molecular design and therapeutic targeting, the environmental implications of pharmaceutical use remain insufficiently integrated into early-stage design frameworks and regulatory standards of early drug development [
4].
Modern drug development follows a highly regulated and sequential pathway, typically spanning 10–15 years from target identification to post-market surveillance [
2]. Following target validation and lead optimisation, candidate compounds undergo extensive pre-clinical testing to evaluate toxicology, pharmacokinetics, and pharmacodynamics [
5]. Only a small proportion of these candidates progress into clinical development, which is divided into Phase I (safety and tolerability), Phase II (dose optimisation and preliminary efficacy), and Phase III (large-scale efficacy and safety evaluation) trials prior to regulatory approval [
3,
6]. However, this standard systematically prioritises clinical endpoints while largely overlooking the downstream environmental fate of pharmaceuticals and their metabolites. Consequently, properties such as persistence, bioaccumulation potential, and ecotoxicity are rarely embedded as primary design constraints.
Following administration, pharmaceuticals are subject to complex absorption, distribution, metabolism, and excretion (ADME) processes that govern both therapeutic bioavailability and elimination pathways. Hepatic metabolism, largely mediated by cytochrome P450 enzyme systems, typically generates more hydrophilic metabolites that facilitate renal or biliary excretion [
7]. Nevertheless, many widely prescribed compounds exhibit incomplete metabolism, resulting in the excretion of parent compounds or biologically active metabolites [
8,
9]. Compounds such as carbamazepine, diclofenac, and ciprofloxacin exemplify this phenomenon, demonstrating high metabolic stability and resistance to biodegradation. These characteristics are intrinsically linked to their physicochemical properties—including aromaticity, halogenation, and structural recalcitrance—which not only impede metabolic transformation but also extend environmental persistence and bioactivity beyond therapeutic contexts.
Upon excretion, pharmaceutical residues enter municipal wastewater systems via multiple pathways, including human excreta, topical wash-off, improper disposal practices, hospital effluents, and industrial discharges [
10,
11]. Wastewater treatment plants (WWTPs) therefore function as critical control interfaces for the environmental dissemination of pharmaceutical contaminants. However, conventional treatment systems were not designed to target trace organic micropollutants, resulting in highly variable removal efficiencies that depend on compound-specific physicochemical properties and treatment configurations [
12,
13]. As a result, many pharmaceuticals undergo only partial transformation, leading to their release into surface waters or accumulation in sewage sludge [
14].
Within WWTPs, pharmaceutical compounds can actively disrupt biological treatment processes and influence microbial ecosystem dynamics. Antimicrobial and enzyme-inhibitory agents may impair microbial consortia responsible for organic matter degradation, thereby compromising treatment efficiency [
15,
16]. Furthermore, the partitioning of hydrophobic compounds into biosolids facilitates their transfer to terrestrial systems via sludge application to arable land, thereby extending their environmental footprint [
11]. Residual pharmaceuticals have been widely detected in aquatic environments at trace concentrations, where chronic exposure has been associated with sub-lethal effects, including endocrine disruption, behavioural modification, and the propagation of antibiotic resistance determinants [
12,
17]. These processes illustrate the complex interplay between pharmacological function, environmental persistence, and ecological impact.
Collectively, these observations underscore the necessity of reconceptualising pharmaceutical development within an integrated framework that aligns therapeutic efficacy with environmental sustainability. This work examines the link between pharmaceutical compounds, human metabolism, wastewater treatment processes, and environmental dissemination, situating these interactions within a One Health perspective, with examples from antibiotics, anticonvulsants, antidepressants, beta-blockers, and non-steroidal anti-inflammatory drugs (NSAIDs).
This entry is adapted from the peer-reviewed paper 10.3390/encyclopedia6070159
