People will learn their doctor has left a practice with little or no explanation, and without a forwarding address. Michael A. Cassidy, chairman of the health law practice group for the Downtown law firm Tucker Arensberg, represents more than 500 local physicians. He said most physician contracts now contain clauses that prohibit doctors from soliciting patients if they leave a practice. Contracts have become more restrictive, but that's not the only reason that more such incidents may occur. The competition between Highmark and UPMC for doctors, and healthcare overhaul that is steering doctors into larger systems may help make these situations more frequent.