MONTPELIER -- The Senate on Tuesday passed a final version of a bill about drugged driving after once again tweaking the definition of driving under the influence of drugs.

The Senate version of H.501 now heads to the House. That body will decide whether to agree with the Senate version or hash out the differences in a conference committee.

The Senate changed the bill significantly from the House version.

Public Safety Commissioner Keith Flynn on Tuesday opposed the Senate version, saying it is essentially identical to existing law. Defender General Matthew Valerio said he supports the Senate version because it takes into account people properly taking legally prescribed drugs.

The existing standard for drugged driving is "under the influence to a degree which renders the person incapable of driving safely." Vermont has a different standards for DUI alcohol and drugs.

The Senate on Tuesday endorsed a drugged driving standard of impairment that "interferes with a person's safe operation of a vehicle."

"Essentially, it's just a modification of the law that's there now, which is in itself virtually unenforceable," Flynn said.

The House version, meanwhile, says a person can be charged if drugs affected him or her "in the slightest degree." That is the change prosecutors want because they say the standard in current law is so high it is nearly impossible to enforce.

The Senate on Monday, just before giving their version final approval, removed the words "in the slightest degree" from the end of the "interferes with a person's safe operation of a vehicle" standard.

Valerio said the Senate language takes into account people who take medications that do not affect, or improve, their ability to drive safely.

"What the drugged driving legislation should do is treat people fairly who have taken prescription medication and can operate their vehicle safely and this does that," Valerio said.

Valerio declined to comment on whether the new version is essentially the same as current law.

Sen. Joe Benning, R-Caledonia, vice chairman of the Senate Judiciary Committee, which vetted the bill, said the new standard is a tweak but still maintains a nexus between unsafe driving and drugs, something that was important to the committee.

The Senate Judiciary Committee voted 3-1-1 in favor of that amendment, Benning said. Chairman Sen. Dick Sears, D-Bennington, said he voted no because he is concerned with how courts might interpret the law without the "to the slightest degree" phrase.

Sears also said he wanted to clarify a point he made Monday about drug recognition experts, specially trained police who perform a 12-step test to determine whether someone is under the influence of drugs.

Sears said he has concerns about when and which drivers are referred to drug recognition officers, and whether there are enough of them in Vermont, not with whether their tests are scientific.