Most cannabis sold on the streets of Madrid is contaminated with dangerous levels of faecal matter, a study says.

Traces of E.coli bacteria and the Aspergillus fungus were found by analysts who examined 90 samples bought in and around the Spanish capital

The samples that were wrapped up in plastic "acorns" were the worst offenders, reportedly because of the way they are smuggled into the country.

Some 40% of these also had the aroma of faeces, the study's lead author said.

Buying, selling and importing cannabis is against the law in Spain, as is using it in public - although it is technically legal to grow it for personal use, provided it is not publicly visible, and to consume it in private.

How was the study carried out?

José Manuel Moreno Pérez, a pharmacologist from the Universidad Complutense in Madrid, collected samples of cannabis directly from street dealers, both in the city and the surrounding suburbs, to determine whether the drugs sold were suitable for human consumption.

His research team then separated the contaminated samples by shape, with some of them resembling "acorns" and others "ingots", to see if one shape had more contaminates than the other.

The study, co-authored with Pilar Pérez-Lloret, Juncal González-Soriano and Inmaculada Santos Álvarez, has been published in the journal Forensic Science International.

What were the findings?

They found that 93% of the acorn-shaped samples contained dangerous levels of E.coli bacteria, as did 29.4% of the ingot samples.

Some 10% of the cannabis samples were also contaminated with Aspergillus, a dangerous fungus that can cause serious health problems.

Most of the samples tested - 88.3% - were not suitable for consumption.

Coloured scanning electron micrograph of Escherichia coli (E.coli) 0157:H7

Mr Pérez later explained the contamination - and the smell - to the Spanish newspaper El País.

The acorns, he said, were more likely to be contaminated because of how they were brought into the country - the cannabis is wrapped up in small plastic pellets and swallowed before the drug smugglers then "take a laxative and expel" them in a toilet. These are then sold by dealers.

Apart from being unpleasant, what are the health risks?

According to the study, the risks associated with E.coli and Aspergillus are serious enough to make the illegal street vending of hashish "a public health issue".

The E.coli infection, for example, can cause diarrhoea, vomiting, stomach pains, fever, and blood in faeces - and for some people this can then lead to even more serious conditions.

Meanwhile, inhaling Aspergillus mould can cause serious problems for people who already have lung conditions, like asthma or cystic fibrosis, or in people who have low immunity.

The study says this is particularly dangerous for cancer patients, who sometimes smoke cannabis to help with the symptoms of chemotherapy.

"These patients have a weakened immune system, meaning that an infection caused by the consumption of contaminated or adulterated hashish could be fatal," it adds.