You may be able to legally use marijuana for medical andrecreational purposes in some states, but the drug is still notwelcome in the workplace. And federal laws and regulations stillrequire that employers keep illicit drugs out of the workplace,state laws notwithstanding. Employers across the country are alsoconcerned that substance-using employees may harm themselves andothers, so there are several incentives to maintain drug-freeworkplaces.

But keeping marijuana and other drugs out of the workplace iseasier said than done. More than 9% of people over the age of 12reported using illicit drugs in the past month, up from 8% in 2007,according to the 2013 National Survey on Drug Use and Health. Andthe percentage of American workers testing positive for illicitdrug use in employers’ urine tests increased in both 2013 and 2014,according to Quest Diagnostics.

Given these trends, it’s no surprise that many employers haveintroduced zero-tolerance drug policies in their workplaces. Butimplementing such policies can be challenging.

Here are three questions you should ask to help ensure youranti-drug policies are as effective as possible.

1. When should we test?

That depends on what your ultimate objective is. Are you seekingto screen out potential hires who use drugs? Are you seeking todeter drug use among all employees? Do you want the ability to testemployees who you suspect are using drugs in the workplace? Do youwant to know whether drug use contributes to on-the-jobinjuries?

The way you answer those questions will determine how you shouldtest your employees. Among other options, employers can considerpre-employment testing, random or periodic testing, and post-injurytesting.

2. How should we test?

Employers have several testing methods to choose from. Urinetesting is the mostly frequently used method; it can detect avariety of drugs in an employee’s system and it’s inexpensive andeasy to administer. But other methods—including oral/saliva andhair testing—may be more appropriate for some employers and incertain circumstances. For example, hair testing can detect druguse for up to 90 days, which may make it more attractive toemployers performing pre-employment testing.

3. How do we stay compliant?

Determining when and how to test should be considered in thelarger context of ensuring legal and regulatory compliance ofoverall zero-tolerance policies. Your labor and employment counselshould help you make sure that your approach to preventing drug usecomplies with all applicable federal, state and local laws andregulations. Among other things, it’s important that you have aclearly written policy that explains to employees the consequencesof a positive test result, and that your policies specificallyaddress medical and recreational use of marijuana—especially instates where such use has been legalized.

As more states legalize marijuana for medical or recreationaluse, or both, employers may find maintaining safe workplaces morechallenging—especially while trying to balance employees’individual rights. Starting with these three key questions shouldat least put you on a path toward meeting those challenges.

Editor's note: Tom Ryan is a managing director and MarketResearch Leader in Marsh’s Workers’ Compensation Center ofExcellence. He is responsible for developing market research,insight, and other content on emerging issues, trends, regulatory,and other changes that affect the workers’ compensation market.This article was first published on Marsh’s website.