Radical Prescription describes the power of the co-pay:
A recent Rand study of 25 large firms found that raising the co-pay for pharmaceutical claims by just $5 reduced yearly drug costs per worker by $163.
It can backfire though:
Higher co-pays can cause consumers to cut back on prophylactic and maintenance medicines. Pitney Bowes, for example, found that high-prices caused their diabetic and asthmatic workers to take their medicines irregularly resulting in sudden and expensive attacks. So Pitney Bowes took a counter-intuitive strategy — to save money they would pay for more of their workers prescriptions.