Drivers are criminally liable when they drive under the influence of any substance. A .08 blood alcohol level is evidence enough where impairment is presumed, and it does not matter whether the driver has been affected by the alcohol or not. However, the driver can still be drunk and responsible for the accident if they had a BAC below that. This is another way of saying that impairment can be a basis of liability, no matter what substance the driver ingested.
Motorists cannot legally take any kind of drug before they get behind the wheel. Even if they took legally prescribed drugs, they can still be responsible for an accident if they were under the influence. The same goes for cannabis products, even if the driver legally took them. If a driver caused an accident when they were under the influence, police can administer a test to figure out whether they had drugs in their system.