Can I require my employee to take medical leave?

Yes, if their health condition is a danger to themselves or others.  You can also require a doctor’s release for them to return to work.  However, be very careful that you are not discriminating against an employee based on the employee’s health condition.  In California, HIV, AIDS and cancer patients, among others, are protected against discrimination based on their health condition.  Also note, your employee may not be aware that he or she is entitled to a leave of absence, and that California’s short term disability insurance (SDI) will cover a percentage of their wages while they are on leave.  Sometimes just informing the employee of their entitlement solves the problem.