Complex sentence: Based on studies at the time was believed to be a strong association with cancer and a fairly low usage outside of North America and several countries have banned the sale (and in some cases the import) of dipping tobacco.
Split into simpler sentences: Based on studies at the time, there was believed to be a strong association of dipping tobacco with cancer, and a fairly low usage outside of North America. Several countries have banned the sale (and in some cases the import) of dipping tobacco.