Seedsman Blog


The widespread prohibition of cannabis can have dramatic consequences in terms of public health. Indeed, forced to obtain their weed via the black market, consumers who do not produce their own cannabis often find themselves in the presence of poor-quality products that can be downright dangerous for their health. This is the conclusion of a study carried out in the streets of Madrid.

Obtaining cannabis on the black market in the streets of Madrid

Spain is one of the most liberal countries in Europe when it comes to cannabis. Self-production is allowed, but it is still forbidden to buy or sell cannabis. However, not all weed users have the means, desire or experience to grow the weed they consume: hence the black market in cannabis and its derivatives, such as hashish. The illegality of this practice leads to major risks in terms of public health, as none of the products sold on the streets are subject to quality controls by the health authorities.

Nevertheless, their consumption remains widespread, including by therapeutic consumers with weakened immune systems. The Spanish media often report large seizures of cannabis or hashish in impressive police operations. But there is little information about cannabis bought on the streets of Spain’s major cities.

Dr Manuel Pérez Moreno analysed these cannabis products in collaboration with the University of Madrid, the Alfonso X el Sabio University, the University of Extremadura and the CANNA Foundation[1]. Forty samples were obtained on the black market from 18 of the 21 districts of the Spanish capital. Samples were obtained following the steps of an average consumer, whether occasional or regular: locating a point of sale, making contact with the seller, observing the sample, paying for it.

The quality of cannabis sold on the black market

Pérez Moreno’s conclusion is clear. Nearly two thirds of the products tested are unfit for consumption. 35% of the samples had a ‘disproportionate’ amount of Penicillium. Prolonged exposure to this fungus or its consumption in large quantities can cause hypersensitivity reactions and even pneumonia, especially in immunocompromised people. 32.5% of the products analysed contained excessive amounts of Mucor, a fungus that is common in the nature and proliferates particularly in humid environments.

Pérez Moreno concludes that most of the cannabis sold on the black market in the streets of Madrid was grown in unsuitable environments, with poor ventilation and inadequate humidity and temperature conditions and/or that the drying process of the plant was not done properly and/or that the samples were packed before being properly dried. Clearly, at some point in the supply chain, minimum hygienic conditions were not met.

In terms of public health, the problem is twofold. On the one hand, as Pérez Moreno points out in El Pais, ‘the day this fungus changes and is replaced by another, Madrid’s emergency services will be full to bursting.’ On the other hand, the number of therapeutic cannabis consumers is growing all over the world. If patients with weakened immune systems consume cannabis of such poor quality, their situation could be made much worse. The problem is even more worrying, as the analysis of the forty samples also revealed an upward trend in THC levels, with an average level of 15%, and a virtual absence of CBD.

Unfortunately, there are no studies prior to Pérez Moreno’s that show a change in the quality of the cannabis circulating on the Madrid black market over time. In any case, prohibition can only lead to this type of drift, which is allowed by the total absence of control. Pérez Moreno’s alarming conclusions are reminiscent of the damage caused by the marketing of synthetic cannabis in Besançon (France, Doubs) earlier this year. On the black market, anything and everything circulates, hence the interest in self-production from cannabis seeds, a practice that allows the grower-consumer to control what he consumes and makes him responsible for his own consumption.

[1] M. Pérez Moreno, Adulteración, contaminación y concentración de principios psicoactivos de la resina de cannabis consumida en la Comunidad de Madrid, doctoral thesis, Universidad Complutense de Madrid, 2019,

Cultivation information, and media is given for those of our clients who live in countries where cannabis cultivation is decriminalised or legal, or to those that operate within a licensed model. We encourage all readers to be aware of their local laws and to ensure they do not break them.


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