A fifth of small-business owners say they would allow an employee with a medical marijuana prescription to use the drug while at work, according to a survey released Wednesday by insurance company Employers Holdings.

The survey, which polled 500 small-business owners with fewer than 100 employees, also found that 42 percent don’t have written policies prohibiting employees from possessing, using or being under the influence of marijuana, and 74 percent don’t require employees to take drug tests.

Employers in the 23 states plus the District of Columbia that have legalized marijuana for medical use (four also allow it for recreational use) have to consider how to balance employees’ right to use the drug with maintaining safe workplaces.

In Illinois, where dispensaries are preparing to open to patients soon, employers are not required to allow medical marijuana patients to use the drug at work and may penalize card-carrying patients for failing drug tests.

The Illinois Department of Public Health has issued 3,100 approval letters to qualifying patients.

Nearly two-thirds of small-business owners polled in the survey said they would not allow marijuana use at work. Seventeen percent said they weren’t sure if they would.

But more than 80 percent said they were unconcerned about employees coming to work high. A tenth said employees have shown up to work under the influence of drugs, alcohol or painkillers.

Employers Holdings, which specializes in insuring small businesses, recommended employers institute a drug-free workplace with clear guidelines stating what is prohibited and what the consequences would be of violating the rules.