Around two thirds of Viagra consumed in the Netherlands may be illegal, warn experts

In a letter published on bmj.com, they say the consumption of illicit drugs might dwarf consumption of legitimate versions -- and they call for the further inquiry into the apparent success of rogue online pharmacies.

It follows reports of a record number of fake drugs seized under Operation Pangea VI, which the authors say "is just the tip of the iceberg.

They therefore set out to estimate the actual use of illicit sildenafil using sewage epidemiology -- an established science for monitoring drugs of abuse.

They measured sildenafil levels at sewage treatment plants serving three cities in the Netherlands (Amsterdam, Eindhoven and Utrecht) over a seven-day period.

Consumption of legitimately dispensed sildenafil was estimated from the national dispensary database 12 months prior to the study until three months after.

They found that at least 60% of the drug could not be explained by dispensing records (Amsterdam 61%, Eindhoven 79% and Utrecht 66%).

"Despite major differences in tourism and commuting, the illicit fraction is similar for each city," say the authors. "Consequently, the unexplained fraction is primarily ascribed to the use of illicit sildenafil."

If this is representative of other communities, "consumption of illicit erectile dysfunction drugs might dwarf the consumption of the legitimately dispensed versions," they warn.

The apparent success of rogue online pharmacies would be an important area of further inquiry, they conclude.