Introduction

We usually think of a medicine’s journey as ending once it has treated the patient. But what happens to a drug after it leaves the body? Medicines have become an essential part of both human and veterinary healthcare, helping to prevent, manage and treat a wide range of diseases. Yet their journey does not necessarily end after they have produced their therapeutic effect.

Following administration, a proportion of a medicine may be excreted through urine or faeces either as an unchanged active compound or as metabolites, allowing pharmaceutical residues to enter wastewater systems [1]. Unused or expired medicines may also enter the environment when they are disposed of incorrectly [1], while hospital use and pharmaceutical manufacturing can provide additional access to the environment. Some pharmaceutical compounds may then pass through wastewater treatment and reach rivers and other aquatic environments [1].

This is not simply a local concern. Pharmaceutical residues have now been detected in rivers across many parts of the world [2]. Their presence raises questions about possible effects on aquatic organisms, ecosystems and antimicrobial resistance. Understanding how medicines enter the environment is therefore an important first step towards making healthcare more environmentally sustainable.

From the Patient to the Environment

One of the major routes of pharmaceutical pollution begins with the normal use of medicines. After administration, not all of a drug is completely broken down by the body. A proportion may be excreted through urine or faeces either as an unchanged active compound or as metabolites [1]. These residues can then enter household sewage and travel to wastewater treatment plants. Once flushed away, these pharmaceutical residues enter the household wastewater system and travel through the sewage network to a municipal wastewater treatment plant [1]. However, conventional wastewater treatment plants were not specifically designed to remove pharmaceutical compounds [1], [3]. Depending on the drug and the treatment process used, some residues may be removed, while others can remain in the treated wastewater and eventually be discharged into the receiving environment [3]. This means that even medicines that are taken correctly can unintentionally become a source of environmental contamination.

Unused Medicines: Another Route into the Environment

However, excretion is not the only way medicines enter the environment. Some pharmaceuticals reach waste and wastewater systems without ever being taken by a patient. Medicines may remain unused because a patient’s condition improves, treatment is changed, or the medicine expires before it can be used. When these unwanted medicines are flushed down the toilet or poured into a sink, their active ingredients can enter sewage directly and may eventually reach aquatic environments if they are not completely removed during treatment [1]. Medicines discarded with general household waste can also contribute to environmental contamination, particularly when pharmaceutical residues reach landfill leachate [1].

This issue is relevant in Aotearoa, New Zealand. A survey of 452 New Zealanders found that 62% had unwanted medicines in their homes, while only around 13–24% of unused medicines were returned to pharmacies, depending on the formulation [4]. These findings suggest that improving public awareness and providing convenient medicine-return options could reduce the amount of pharmaceutical waste entering the environment. Households, however, are only one source of pharmaceutical pollution. Hospitals and pharmaceutical manufacturing facilities can also release pharmaceutical residues into waste streams.

Hospitals and Pharmaceutical Manufacturing

Hospitals can also act as point sources because a wide variety of medicines are used within a relatively concentrated setting. After administration, pharmaceutical residues may enter hospital wastewater through patient excretion or the disposal of medicines and eventually reach municipal wastewater treatment plants [5]. However, the contribution from hospitals varies between drugs. One study found that hospital discharges generally contributed less to the overall pharmaceutical load than municipal wastewater [5].

Pharmaceutical manufacturing can create a different problem. Wastewater produced during drug manufacturing may contain active pharmaceutical ingredients, and inadequate control of these discharges can result in particularly high concentrations in the surrounding environment [6]. A study of wastewater from a treatment plant serving around 90 bulk drug manufacturers in India reported exceptionally high concentrations of several pharmaceuticals [6]. Further investigation in the same production region detected pharmaceutical compounds in surface water and groundwater, demonstrating that contamination associated with manufacturing can extend beyond the immediate discharge site [7]. Together, these pathways show that reducing pharmaceutical pollution requires attention to the entire life cycle of a medicine, from its production to its final use and disposal [6], [7].

Can Wastewater Treatment Remove Pharmaceuticals?

It might seem reasonable to assume that once pharmaceutical residues reach a wastewater treatment plant, they are simply removed before the water is released back into the environment. However, conventional wastewater treatment plants are primarily designed to remove contaminants such as organic matter, nutrients and pathogens, rather than specifically targeting pharmaceutical compounds [3]. Once pharmaceuticals enter a treatment plant, their fate can vary considerably. Some compounds may be degraded by microorganisms or removed with sewage sludge, while others can remain in the treated wastewater and be discharged into the receiving environment [3].

Importantly, removal efficiency differs greatly between medicines. In one study of a municipal wastewater treatment plant in Germany, removal rates ranged from only 7% for one drug to more than 99% for salicylic acid [8]. This means that the effectiveness of wastewater treatment cannot be generalised across all pharmaceutical compounds. A study conducted at a wastewater treatment plant in New Zealand similarly found that the behaviour and removal of pharmaceuticals differed depending on the compound and the treatment process being used [9]. Pharmaceutical residues that survive treatment may therefore enter rivers, estuaries and other receiving waters through treated wastewater discharge. The fact that pharmaceutical residues can survive wastewater treatment raises another question: what happens when these biologically active compounds enter aquatic ecosystems?

Why Does Pharmaceutical Pollution Matter?

Pharmaceutical residues are designed to produce biological effects, so their presence in the environment can potentially affect organisms other than the patients they were intended to treat. One well-known example involves the synthetic oestrogen 17α-ethinylestradiol (EE2), which is used in hormonal contraceptives. In a whole-lake experiment in Canada, long-term exposure to environmentally relevant concentrations of EE2 disrupted reproduction in fathead minnows and ultimately caused a severe decline in the fish population [10].

Psychoactive medicines may also influence aquatic animals because many biological signalling systems are shared across species. In one study, European perch exposed to the anti-anxiety medicine oxazepam at concentrations found in wastewater-affected surface waters became more active, less social and showed increased feeding behaviour [11]. These behavioural changes may appear subtle, but changes in feeding, movement and social behaviour can potentially alter interactions between organisms within an ecosystem.

Antibiotic residues create another concern because even relatively low concentrations can place selective pressure on bacterial populations. Laboratory experiments have shown that concentrations several hundred times lower than the minimum inhibitory concentration can favour resistant bacteria over susceptible bacteria [12]. This does not mean that environmental antibiotic pollution is solely responsible for antimicrobial resistance, but it suggests that contaminated environments may contribute to the selection and maintenance of resistant bacterial populations.

Towards More Sustainable Medicine Use

Completely preventing pharmaceuticals from entering the environment would be unrealistic, as medicines remain essential for human and animal health. Instead, reducing pharmaceutical pollution requires action at several stages of a medicine’s life cycle. One relatively simple approach is to prevent unused or expired medicines from entering sewage or household waste in the first place. Medicine take-back schemes and clearer disposal guidance can provide safer routes for unwanted medicines and reduce inappropriate disposal [1], [13]. OECD(Organisation for Economic Co-operation and Development)  recommendations also emphasise approaches that address pharmaceutical pollution at its source rather than relying only on removing residues after they have already entered the environment [13].

Wastewater treatment provides another opportunity for intervention. Additional treatment technologies such as ozonation and activated carbon can remove many pharmaceutical micropollutants more effectively than conventional treatment alone [14]. However, their effectiveness can differ between compounds, and introducing advanced treatment also requires additional infrastructure, energy and financial investment. This means that wastewater treatment should form part of a wider strategy rather than being treated as the only solution.

Pharmaceutical manufacturers also have a role to play. In 2024, the World Health Organisation released guidance specifically addressing wastewater and solid waste from antibiotic manufacturing, including measures intended to limit antibiotic pollution and reduce environmental conditions that may contribute to antimicrobial resistance [15].

Ultimately, sustainable healthcare does not mean restricting access to necessary medicines. Instead, it means gaining their health benefits while reducing avoidable environmental exposure throughout their production, use and disposal.

Conclusion

Medicines play an essential role in protecting human and animal health, but their journey does not necessarily end after treatment. Pharmaceutical residues can enter the environment through normal excretion, the disposal of unused medicines, healthcare facilities and pharmaceutical manufacturing. Once these compounds reach wastewater systems, some may survive treatment and enter rivers and other aquatic environments, where they can potentially affect wildlife and contribute to wider concerns such as antimicrobial resistance.

Reducing pharmaceutical pollution therefore requires action throughout the entire life cycle of a medicine, from responsible manufacturing and appropriate use to safe disposal and improved wastewater treatment. The aim is not to reduce access to medicines that people need, but to ensure that their benefits to health do not come with avoidable environmental consequences. Perhaps the end of a prescription should not be considered the end of a medicine’s story.