A related claim, especially in the UK, is that the cannabidiol/THC "ratio" has decreased over the past few decades, resulting in a new and presumably more dangerous form of cannabis that never existed before (since CBD is thought to attenuate some of the negative side effects of THC). While there is little to no reliable data before 2005 on such ratios in the UK, making comparisons to the past impossible, the US data going back to the 1970s shows little to no clear trend, and there have always been strains with extremely low ratios. Ratios are also known to vary widely between strains and growing/harvesting methods.