Laws on this topic vary from state to state.
As a general rule, in this situation the employee is responsible for the cost of the drug (and also alcohol) rehabilitation. If the employee’s health insurance policy includes substance abuse rehab, then all or part of the cost may be covered by the health insurance.
In some states, if you do drug testing and your testing identifies an employee with a drug or alcohol abuse program, you are required by law to pay part or all of the rehabilitation costs.
Also in some states, state law strictly sets out conditions you must follow to do drug testing. However, the cost of rehabilitation is at the employee’s expense if not covered by health insurance.
On a related topic: Employees who use illegal drugs are not automatically considered disabled for purposes of protection under the Americans with Disabilities Act (ADA). Alcoholism is considered a disability under ADA. Some state laws protect drug addicts and alcoholics as disabled. Know your state law in this area and use caution before taking any disciplinary action against employees you believe might be drug abusers or alcoholics.