Ex-cop Neil Colleran is confident that he can get a lot of drivers off the hook for drunken driving charges. He isn't thinking, though, of fixing tickets or sweet-talking a judge.

Colleran, a practicing attorney, is president of Med-Law Corp., a new company that is offering Massachusetts drivers an on-the-spot blood test, the results of which can be used as a second opinion to the potentially inaccurate Breathalyzer tests used by police. If the blood analysis comes out in his favor, the driver may be able to beat the rap -- no small matter in Massachusetts, which has some of the toughest drunken driving laws in the country. A conviction can mean stiff fines and loss of the offender's driver's license.

But Second Opinion, as the Cambridge, Mass., company calls its product, will be a dicey proposition for some drivers. If the blood test confirms the Breathalyzer results, the prosecution can attempt to use the test as additional evidence against the driver. Second Opinion, then, is not for celebrants who belt down drink after drink at the local watering hole.

The market opportunity for Med-Law exists because of the long-standing controversy over Breathalyzers. "They're defective, they're unreliable, they're wrong way too much of the time," Colleran says. "And I mean all of them, from the 20-year-old models to the latest state-of-the-art." He adds: "Blood doesn't make mistakes. An arrested motorist is entitled by law to a blood test. We're going to see that he gets it."

After flunking the Breathalyzer test, a driver can call a toll-free number to summon a Med-Law nurse to the police station. The pinprick blood sample is rushed to a lab, which completes the analysis within three days. Med-Law's fee is $150.

To reach potential customers, Med-Law is placing about 13,000 signboards, complete with pamphlets and wallet-size cards, in bars, liquor stores, and restaurants in Massachusetts. "That's just the beginning," he said. "We're going across the country with this."

Stephen M. Limon, an assistant district attorney in Massachusetts, however, doubts that a large market exists for Second Opinion. "The problem is that there's a great need for it, but on the part of very few people," he says. "The sober motorist who fails a breath test exists all right, but he doesn't show up in the average police station all that often."

And, of course, Med-Law faces the risk that engineers may develop a better Breathalyzer. If that happens, it could be Colleran's company that ends up with a headache and a hangover.