Refer previous comments from
@Jason Simmons they have a blood test, because he was taken to hospital, they can get a forensic medical or something done on the test to give an assessment on whether the level of drugs in his system may have impaired his driving.
Drugs impair your fine motor skills, regardless of ability to mask said impairment. That is the starting point of all legislation relating to being under the influence of something and driving a vehicle, horse, bicycle, whatever.
Alcohol has known effects on the body regardless of alleged “tolerance” to alcohol (usually ego related) and is eliminated from the body at a more or less consistent rate. Hence why there are various percentage levels for alcohol. 0.05%, 0.150% and so on. Medical people can describe in detail the medical effects on the body when it is affected by Acetaldehyde (metabolised byproduct of ethanol) these effects are consistent and occur independently of so-called “tolerance” so courts can be assured of the reliability of such evidence.
Illicit ‘dangerous’ drugs such as Cocaine, Methylamphetamine and so on however do not. Not as consistently or reliably as Ethanol. They also widely vary in chemical composition, strength (purity) and other factors.
This is why there is an offence for ‘driving with a relevant drug present”. Because medically speaking it is nigh on impossible to determine any reliable level of impairment for ‘low’ levels of such drugs. But such drugs ‘do’ cause impairment and so legislatures have decided to make driving under such circumstances illegal, albeit it is a very minor offence and punished far less heavily than even low ranged drink driving offences in the main.
However, when a blood sample is taken to determine quantities of such drugs in a person’s system, the toxicology results that come back in the form of an evidentiary certificate, list quantities and percentages of detected drugs present.
To determine whether the detected level of drugs in the system would result in the suspect being deemed “under the influence” the police would (or should) have sought an “expert” medical opinion from a forensic medical officer (FMO).
The drug certificate would have been provided, along with basic information on the suspect’s age, gender, height / weight, history and the observed level of indicia (how munted) the suspect was would (or should) also have been provided by the investigating police. Such might have included - manner of driving, slurred (or otherwise) speech, any conversations held (aka what was said) fixed / dilated pupils, flushed appearance, behaviour / acts at the scene, steadiness on their feet, etc.
Add all that together and you get an evidentiary basis to charge (or not) for ’under the influence’.