Medication Residues in Our Waterways: Causes and Remedies

From Pharmaceutical Use to Wastewater

When we take medication, our body processes some of the active substances. The liver converts many drugs into other compounds, which are then mainly excreted through urine and feces. Depending on the medication, up to 90% of the administered dose, in its original or converted form, can end up in wastewater.

But there are also other routes. During showering or washing, residues of medicinal gels, creams, and ointments rinse off directly from the skin. Additionally, active substances end up in wastewater when unused or expired medicines are disposed of via the sink or toilet.

All these routes ensure that medicines and their breakdown products find their way through the sewage system into our wastewater treatment plants.

Why Traditional Water Treatment is Insufficient

According to the Flemish Environment Agency's data from 2025, approximately 88% of household wastewater in Flanders is treated. This is a significant improvement compared to 1992, when the treatment rate was only 29%.

Traditional wastewater treatment largely relies on biological processes in which microorganisms break down organic pollutants. However, many medications are difficult or barely biodegradable. As a result, even in treated wastewater, not all pollutants are removed.

Moreover, the removal efficiency varies significantly from substance to substance. For instance, over 90% of the paracetamol present is removed, while for diclofenac (an anti-inflammatory painkiller), the average is only 30%. Other medications are particularly persistent. An example is carbamazepine, used for conditions including epilepsy. This substance is difficult to break down in wastewater treatment plants and in the natural water environment.

As a result, some of the pharmaceutical residues leave the treatment plant with the treated wastewater and enter rivers and streams. Additionally, there is a direct intake of untreated wastewater. In Flanders, approximately 12% of household wastewater is not treated. During heavy rainfall, sewer overflows can also temporarily discharge untreated wastewater into watercourses.

What happens to pharmaceutical residues in surface water?

Once in the surface water, pharmaceutical residues undergo various processes. Sunlight and microorganisms break down some substances further, but others remain present for a long time. Certain substances attach to silt particles and can end up in the sediment, where degradation often occurs more slowly.

The breakdown of a pharmaceutical does not necessarily mean that the environmental problem disappears, as new compounds can be formed during the conversion, which may themselves be biologically active or toxic. Conversely, substances that have been converted into an inactive form within the human body can under the influence of bacterial enzymes revert to their original active form.

Exactly what effects these conversions have on aquatic life is still insufficiently known. The effects of degradation products and their interactions with other substances particularly require further research.

Low concentrations can have significant consequences

Pharmaceutical residues are often present in very low concentrations in water, sometimes not more than a few micrograms or nanograms per litre. Yet, even in these small amounts, they can be harmful to aquatic organisms.

Medicines are developed to influence specific biological processes. Because some of those processes also occur in aquatic organisms, pharmaceutical residues can inadvertently disrupt their health, behaviour, or reproduction. Some examples from scientific research make this clear: 

• Hormonal medicines affect the reproduction of fish. The synthetic oestrogen ethinylestradiol can lead to feminisation of male fish even at concentrations of about 1 nanogram per litre. Research has also shown that long-term exposure can have consequences for entire fish populations.
• Painkillers and antidepressants have other effects. Diclofenac can cause damage to the gills, liver, and kidneys of fish. The antidepressant fluoxetine can change the behaviour of fish and crustaceans. 
• Antibiotics pose an additional risk. They can inhibit the growth of algae and microorganisms and thereby disturb the ecological balance. Moreover, low concentrations of antibiotics in surface water, sediment, and wastewater treatment plants can contribute to the selection and spread of antibiotic-resistant bacteria.

Therefore, the impact of pharmaceutical residues depends not only on their concentration but also on the properties of the substance and the sensitivity of the organisms exposed to them.

Drinking water sources are also under pressure

Medicine residues are not confined to surface water. They can also end up in groundwater. Since both are important sources for drinking water production, their presence poses an additional challenge for water companies.

Water companies manage to produce safe tap water, but the removal of persistent substances may require additional purification techniques. Activated carbon filtration and advanced oxidation are examples of techniques that can be employed for this purpose. These extra treatments, however, make drinking water production more complex and expensive. Therefore, it is important to limit the inflow of medicine residues earlier in the water cycle.

From source control to additional purification

Measures are needed at different points in the chain to reduce the spread of medicine residues. The first point of impact is the use of medicines themselves. Healthcare providers such as doctors and pharmacists can help prevent unnecessary use and, when medically justified, consider the environmental impact of various medicines. Proper collection of unused or expired medicines also helps prevent active substances from ending up directly in the sewer.p>Targeted measures are also possible in care institutions, where relatively large amounts of medicines are used. For example, patients, after administration of certain contrast agents, can collect their urine separately in special urine bags, preventing these substances from entering the wastewater. Net Zero Healthcare Impact (NZHI) has developed a practical tool and coaching approach that can help hospitals and medical services map out and reduce their total sustainability impact.

Specific purification of hospital wastewater also offers possibilities. In the Netherlands and Flanders, such solutions are being explored, including within the European project PREWAPHARM.

Water quality requires an approach across the entire chain.

Medicine residues clearly show how closely healthcare and water management are linked. Substances intended to treat humans can have unintended consequences for water organisms and the quality of our water sources after use.

Solely focusing on extra purification is therefore insufficient. Prevention, better insight into the spread and effects of medicine residues, and targeted measures at the source are also needed.

This requires cooperation between healthcare providers, water managers, researchers, technology developers, and policymakers. A chain approach that prevents substances from entering the water and simultaneously improves the removal of residual contamination offers the most comprehensive perspective.

This article is based on 'Medicines and medicine residues in water: what impact on aquatic life?' by Dirk Halet, strategic coordinator for Water at VITO Kennispunt Water, and Brecht De Tavernier, emergency doctor at ZAS Antwerp and founder of Net Zero Healthcare Impact. The original article appeared in Huisarts Nu (2026, 55(4)).