All over the country, employers say they see a disturbing downside of tighter labor markets as they try to rebuild from the worst recession since the Depression: They are struggling to find workers who can pass a pre-employment drug test.That hurdle partly stems from the growing ubiquity of drug testing, at corporations with big human resources departments, in industries like trucking where testing is mandated by federal law for safety reasons, and increasingly at smaller companies.But data suggest employers’ difficulties also reflect an increase in the use of drugs, especially marijuana — employers’ main gripe — and also heroin and other opioid drugs much in the news.Drug use in the work force “is not a new problem. Back in the ’80s, it was pretty bad, and we brought it down,” said Calvina L. Fay, executive director of the Drug Free America Foundation. But, she added, “we’ve seen it edging back up some,” and increasingly, both employers and industry associations “have expressed exasperation.”