Complex sentence: Bromism was once a very common disorder, being responsible for 5 to 10% of psychiatric hospital admissions, but is now uncommon since bromide was withdrawn from clinical use in many countries and was severely restricted in others.
Split into simpler sentences: Bromism was once a very common disorder, being responsible for 5 to 10% of psychiatric hospital admissions. Bromism is now an uncommon disorder because bromide was withdrawn from clinical use in many countries and was severely restricted in others.