Do employers always have a right to know about employee drug use?

For this Discussion Board assignment, consider the following statement:

Drug use is information that is rightfully private and only in exceptional cases can an employer claim a right to know about such use.
Post a response to the Discussion Board that either defends or opposes this statement.

Discussion Board Assignment Guidelines:

Determine your stance on the issue of drug testing and employee drug use, and in 3 5 paragraphs, write how you feel about the issue.

You may use the following questions to help outline your response:

Do employers always have a right to know about employee drug use?
What if the drug use has no effect on employee performance?
What if the drug use constitutes a serious health risk?
How would you (as a manager) address the issue of employee drug use?