Can I be forced to take a drug test after an accident?
The first thing to know is that, when a police officer comes to the scene to assess the situation and create a police report after a car accident, drug or alcohol testing of the drivers is not standard protocol. (Of course, special circumstances may exist in your case. For example, if you got into a car accident while "on the clock", you and your employer may have entered into a prior agreement under which employees agree to submit to drug testing after any job-related vehicle accident.)
Having said that, if the officer suspects that one of the drivers involved in the accident may be under the influence of alcohol or drugs, the procedural rules become the same as those for a standard traffic stop.
A number of factors might lead a police officer to legitimately suspect that impaired driving may have played a role in the accident -- one of the drivers is exhibiting slurred speech or reeks of marijuana, for example, or there is a half-empty liquor bottle or half-smoked joint in plain view. So, based on this suspicion (called "probable cause" in legalese) the officer may ask the driver to submit to a Breathalyzer test at the scene, or a urine or blood test at a secondary location.
As with a routine traffic stop, you have the legal right to refuse to submit to such testing. But refusing to submit to the test does not mean that you cannot be arrested for, and later convicted of, driving under the influence (DUI) or driving while intoxicated (DWI), whatever the offense is called in your state. If there is ample independent of your intoxication and its effect on your driving ability, test results may not be necessary.
What's more, your refusal to submit to testing can carry its own consequences under "implied consent" laws, which are in place in every state.
These laws state that, through the act of operating a motor vehicle, drivers have implicitly agreed to submit to chemical testing for drug or alcohol intoxication when a law enforcement officer has a "probable cause" belief that a DUI/DWI has occurred.
So, under implied consent laws, you're free to refuse drug and alcohol testing, but that refusal alone will open you up to penalties such as driver's license suspension and other sanctions (depending on where you live). And since you can still be convicted of DUI/DWI without chemical testing, the sanctions associated with an implied consent law violation can come on top of a DUI/DWI conviction.