Hong Kong’s waters at risk from over-the-counter drug pollution

A recent study of Hong Kong’s river and estuary systems has uncovered an overlooked major source of water pollution: common over-the-counter (OTC) drugs. Researchers from City University of Hong Kong (CityUHK) found that accessible, everyday OTC drugs, such as painkillers, antihistamines, and caffeine, accounted for up to 85% of pharmaceutical pollution in these waters during the wet season—far outpacing prescription-only medicines.

The study was led by Professor Kenneth Mei-Yee Leung, Director of the State Key Laboratory of Marine Environmental Health (SKLMEH), Chair Professor of Environmental Toxicology and Chemistry, and Associate Dean of Science College at CityUHK, in collaboration with the Guangdong Research Institute of Water Resources and Hydropower at Guangzhou in China.

”We often assume that complex or restricted prescription medications pose the greatest environmental risk, but our findings shine a new light on everyday household medicines,” says Professor Kenneth Leung. ”Because OTC drugs are easily accessible and continuously consumed, they act as ‘pseudo-persistent’ pollutants. Even if they break down easily, our constant use means they are always present in our aquatic ecosystems.”

Molecular properties

Pharmaceuticals tend to be mobile in water due to their molecular properties, allowing them to transport through river networks and enter the ocean via estuaries, posing further threats to marine ecosystems. In fact, it is well known that pharmaceuticals are present in rivers, estuaries, and coastal waters worldwide.

”Pharmaceutical pollution is not a local pollution issue at a specific site. Hence, we should adopt a river-estuary-sea perspective to prioritize pollutants for control and management,” Leung adds.

Notably, existing site-specific risk assessments lack a macroscopic perspective, making it difficult to distinguish widespread ecological threats from localized contamination. To address this concern, the research team introduced a newly developed paradigm that could advance the risk assessment.

”Our approach integrates the river/estuary-to-sea transport trajectory modeling with consideration of persistence, mobility, and toxicity (PMT) of the pharmaceutical pollutant and thus the results can better evaluate and prioritize pharmaceuticals for risk management,” explains Leung.

Priority pollutants

They team found that 80% of the investigated pharmaceuticals met the criteria for being PMT pollutants in water environments. Caffeine, paracetamol, cetirizine, cimetidine, sitagliptin, and fexofenadine were identified as priority pollutants for control in the river-estuary system of Hong Kong, exhibiting a greater potential to harm sensitive marine habitats of the endangered Indo-Pacific humpback dolphin. Among those, the top four pharmaceuticals with the highest risks were all OTC drugs.

By pinpointing high-priority pharmaceuticals and the rivers that contribute most to their spread, the team suggests that targeted interventions, such as upgrading sewage treatment and installing stormwater retention tanks along major discharge rivers, could effectively reduce their emissions into the sea.

”Raising public awareness is also essential,” says Leung. ”Residents should return unwanted or expired medications for proper disposal rather than throwing them away or flushing them. Clearer government guidance on safe drug disposal is urgently needed.”