What are the legislative and court restrictions on employer drug testing in both the private and the public sector?

In the US, companies can test employees for drug use. However, under any circumstances they cannot do so. For private companies, drug testing is controlled by individual states. States generally permit testing as long as proper procedures are followed. States sometimes give employers a discount on their employees' compensation insurance premiums provided that they follow certain procedures to keep a drug free workplace. On the other hand, states with restrictive drug testing laws, they don't usually allow testing for drugs except in particular circumstances.