Complex sentence: Cannabis was downgraded from a Class B to a Class C drug in 2004, making most cases of possession non-arrestable, however Rethink wants government support for new research into the relationship between severe mental illness and cannabis.
Split into simpler sentences: Cannabis was downgraded from a Class B to a Class C drug in 2004, making most cases of possession non-arrestable. However, Rethink wants government support for new research into the relationship between severe mental illness and cannabis.