Urban sewage analysis shows difference in cannabis and cocaine consumption between Madrid and Barcelona

Cities also reveal their habits through what flows through sewage systems. In Madrid, a study based on the analysis of wastewater identified a higher presence of cocaine metabolites than in Barcelona, while indicators linked to cannabis appear at significantly lower levels. The research was conducted by Madrid Salud in partnership with the Complutense University and analyzed nearly 400 samples collected between December 2023 and October 2025.

According to the survey, the data allows observing population consumption trends and contributing to the planning of public health policies, a methodology that has been discussed in studies on cannabis effects on the body and population behavior.

Wastewater analysis reveals consumption patterns in cities

According to the report published by the Cáñamo portal, the study analyzed samples from the eight sewage treatment plants in the Spanish capital. The method measures metabolites, compounds eliminated by the body after the consumption of certain substances, detected in wastewater.

These analyses allow estimating consumption patterns on a population scale without identifying individuals.

The results indicate that cocaine and cannabis are the most prevalent illicit substances in Madrid's wastewater. Following are MDMA, methamphetamine, and amphetamine. Scientific research on the behavior of these substances also appears in studies on inflammation and cannabis in anxiety, a topic investigated by international universities.

Madrid surpasses Barcelona in cocaine residues

The data shows that Madrid records 369 milligrams of cocaine per thousand inhabitants per day, a slightly higher number than the 349.4 milligrams observed in Barcelona.

In the case of cannabis, the situation is reversed: Madrid presents 77.2 milligrams per thousand inhabitants daily, while Barcelona registers 220.7 milligrams.

Still according to the Cáñamo portal report, these numbers refer only to the metabolites detected in sewage, not representing a direct count of consumers or individual prevalence. The debate on monitoring psychoactive substances also appears in international discussions on cannabis regulation and reclassification.

Study uses methodological approach for European comparison

The report also highlights that comparative data with other cities were calculated from the average of April 2024 from the eight treatment plants in Madrid. This methodology was adopted to align the results with the baseline used by the European agency responsible for wastewater epidemiology studies.

Although the complete study analyzed the period between December 2023 and October 2025, the comparative overview uses this specific approach for international standardization.

Differences between cities and usage patterns

In comparison with other European capitals, Madrid appears below Lisbon in cocaine consumption and far behind Amsterdam regarding cannabis, MDMA, and ketamine.

The analysis of wastewater also highlights differences in the temporal pattern of substance use. Cannabis consumption tends to occur more evenly throughout the week, while MDMA shows peaks associated with recreational use on weekends.

Studies of this kind are being used in different countries to observe urban trends related to psychoactive substance consumption.

Data complements public health research and indicators

Madrid Salud also presented the results as a complementary tool to other sources of information, such as population surveys, police seizures, and health system data.

According to the Cañamo portal, this combination of indicators helps broaden the understanding of the drug consumption phenomenon in urban environments, offering a metropolitan scale that is not always captured by other monitoring methods.

Source: with information originally published by the Cañamo.