While alcohol is the single largest contributor to fatal car crashes, a new study suggests that other drugs post significant dangers on the road as well.

Researchers found that of U.S. drivers who died in a crash, approximately 25 percent tested positive for drugs.  The most common drugs were marijuana and stimulants, like cocaine and amphetamines.

In the study, which looked at data from 44,000 fatal car crashes from the years 1998 to 2009, marijuana was linked to speeding and seatbelt non-use, while stimulants were linked to all types of crash fatalities—whether from speeding, failure to obey other traffic laws, inattention, or forgoing seatbelts.

A recent government study found that about 14 percent of U.S. drivers tested positively for drugs when randomly pulled over.

However, while all states report the blood alcohol levels of drivers in traffic deaths, only 20 test for drugs.

The White House announced last year that it would be encouraging more states to adopt drugged driving laws.

This would include setting a standard for drug levels that impair driving, and also tackling the issue of drugs that linger in the body for days or weeks after use.

More than a dozen states have drugged-driving “per se” laws, which usually mean zero tolerance for any detectable amount of certain drugs in a driver’s blood or urine.  The specific drugs prohibited vary from state to state.

Until the nation sets a standard, the researchers said, it is important to continue to study how various drugs may impair drivers.

However, the overall takeaway message from the study is, “Don’t drink or don’t consume drugs when you’re going to drive,” said lead researcher Eduardo Romano, Ph.D.

The study was published in Journal of Studies on Alcohol and Drugs,