Since legal commercial sales of recreational marijuana began in Colorado on New Year's Day, my home state seems eager to live up to the reputation pop artist John Denver gave it in 1972 when he wrote “Rocky Mountain High.”

In less than a week, this reefer madness forced some pot shops to begin rationing the product after an estimated 100,000 people shelled out more than $5 million to purchase what had been illegal in this country since 1938 — the year the nation's first pot bust occurred, coincidentally, in Denver.

Sales continue at a clip of $1 million daily, the Denver Post reports.

Colorado and Washington were the first states to legalize recreational marijuana but they certainly won't be the last, as public opinion shifts on the issue.

A recent CNN poll found 55% of those questioned said pot should be legalized.

The Justice Department announced in August that it would not stand in the way of state laws legalizing recreational marijuana use.

Since then, legislatures in Delaware, Hawaii, Maryland, New Hampshire, Rhode Island and Vermont have said they will tackle the issue this year, according to the Washington-based Marijuana Policy Project.

Meanwhile, 20 states and the District of Columbia already have in place medical marijuana laws, considered the precursor to full legalization: Colorado authorized medical marijuana in 2000.

While employers in Colorado still have full authority to impose drug prohibitions on employees, it is only a matter of time before an employee who is fired for legal off-duty marijuana use attempts to litigate the issue, just as some employees have taken their employers to court for restricting their use of tobacco products. More than a dozen states have specific statutes that protect off-duty use of tobacco and other lawful products.

Six states — Arizona, Connecticut, Delaware, Illinois, Maine and Rhode Island — have passed laws that restrict the firing of medical marijuana users.

So far, a Colorado intermediate appeals court has sided with an employer that fired a Colorado worker with a medical marijuana card for testing positive on a drug test, finding that federal law prohibiting cannabis use trumped state law authorizing its use. The Colorado Supreme Court has announced it will review that April 2013 ruling in Coats v. Dish Network L.L.C. — decided six months after passage of the state's ballot initiative legalizing recreational marijuana.

But even if an employer knowingly permits a legal marijuana user to work, it still might incur costly legal expenses if that worker causes injury to a customer, another employee or himself.

Yes, the times they are a-changin', and employers need to be prepared for any potential “pot” holes in the road ahead.