Cannabis potency data are predominantly collected through analysis of police seizures at the national level. As such, they should not be assumed to be representative of the type of cannabis used at the retail level. Law enforcement methods create a risk of bias in selection according to various factors such as degree of criminal involvement, ethnicity, age, gender, and location. By contrast, cannabis potency data reported by the Netherlands (Trimbos Institute) can be assumed to be representative of cannabis available at the retail level, as data are collected using a standardised protocol consisting of random sampling from national retail outlets each year [72]. We recommend that, where possible, additional European countries should sample cannabis from sources other than seizures in order to improve the reliability and validity of the data. As policies towards cannabis are becoming more permissive in Europe, this may facilitate improved monitoring of cannabis potency at the retail level. An additional consideration is that for all countries reporting data on cannabis potency, the reliability of data may be influenced by the sample size of cannabis products used
