Section 6

Drugs

Patients taking mild sedatives or short-acting hypnotics (sleeping pills) who experience no drowsiness (other than predictable sleep enhancement) may still have residual impairment the next day. Use of benzodiazepines is a significant risk factor for unsafe driving, especially among young people and older adults. Patients who are more heavily sedated for therapeutic reasons should be advised not to drive. Concomitant use of alcohol in these situations raises the risk of impairment.

Drowsiness and dizziness are frequent — and unpredictable — adverse effects of older antihistamines, motion-sickness medications, and muscle relaxants, such as carisoprodol or cyclobenzaprine. The newer “non-drowsy” antihistamines are considered safer, but they may have a depressant effect on the central nervous system. Patients using these drugs for the first time must be warned not to drive until it is determined whether they are prone to these adverse effects.

A patient’s use of OTC preparations that may cause dizziness and drowsiness, such as dimenhydrinate, older-generation antihistamines, and muscle relaxants (including products containing any of these agents) needs to be evaluated carefully. Dextromethorphan, a common cough suppressant, has
been shown to cause impairment (lane weaving, failure to obey traffic signals, and involvement in collisions), especially when combined with an antihistamine such as chlorpheniramine.

Euphoria, depression, or inability to concentrate can follow the use of opiates such as codeine (prescription or OTC), heroin, morphine, and synthetic opioids such as meperidine and fentanyl. Patients should be assessed for adverse effects, as well as frequency of use, tolerance, and dependence. Patients on long-term prescribed opioid analgesic therapy should be monitored for adverse effects, especially drowsiness.

Patients on a formal opioid agonist maintenance program of methadone or buprenorphine prescribed by a physician are usually eligible for Classes 5 and 6 drivers’ licences. A waiting period following initiation of an agonist maintenance program is recommended before resumption of driving, and clinical monitoring for concomitant use of other drugs is recommended (e.g., by urine drug screening). Patients in opioid agonist treatment programs may also be eligible for certain commercial licences. Assessment and follow-up monitoring need to be tailored to the individual.

The adverse effects of central nervous system stimulants, such as amphetamines and cocaine, are unpredictable and often impair ability to drive safely. Abuse of these drugs is a contraindication to driving.

Prescription use of amphetamines, such as those used for attention deficit and sleep disorders, may not impair ability to drive if used regularly at a stable dose. However, patients receiving such therapy should be followed regularly by the prescribing physician.

Drugs such as cannabis and its derivatives, lysergic acid diethylamide (LSD), and methylenedioxymethamphetamine (MDMA) alter perception. Driving is contraindicated if any of these drugs is causing impairment. Patients using medical marijuana must be assessed on an individual basis to determine safety related to driving. It is illegal to drive a motor vehicle while under the influence of any drug that causes impairment of the driver’s ability to safely operate a motor vehicle, regardless of whether the drug has been prescribed by a physician.

Inhalants, such as solvents, glue, and gasoline, are toxic to the central nervous system. Use of these inhalants may also result in substance dependence and impairment of the ability to operate a motor vehicle during acute intoxication or because of chronic damage to the brain.

Patients taking antidepressants or antipsychotics should be observed during the initial phase of dose adjustment and advised not to drive if they show any evidence of drowsiness or hypotension. Tricyclic antidepressants have been found to impair driving ability, especially in older adults. Patients who are stable and symptom-free on maintenance doses can usually drive any class of motor vehicle.

Some of the drugs used to control epileptic seizures can cause drowsiness in certain patients, particularly when first prescribed or when the dose is increased. Patients should be closely observed and warned not to drive while this adverse effect persists. Patients taking these drugs may also be restricted from driving because of the underlying seizure disorder. Patients should be advised of the risk of seizure activity and the potential for driving restriction that may occur with dose adjustments. Please see Section 11, Nervous system, in this guide, specifically the alert box at the beginning of the section and subsection 11.4, Seizures.

Patients should be advised not to drive for 24 hours following conscious sedation (see Section 23, Anesthesia and surgery).

Heavy doses of some anti-infective agents or therapeutic doses in some instances may cause drowsiness or imbalance. Patients should be warned not to drive if these adverse effects occur.

Antimicrobial and antiprotozoal agents reported to affect consciousness are amoxicillin, cloxacillin, ticarcillin, cephalothin, cefazolin, ceftazidime, cefuroxime, tobramycin, lomefloxacin, pefloxacin, amphotericin B, acyclovir, chloroquine, clioquinol, and metronidazole.

Anticholinergics frequently cause sedation and delirium (acute onset of cognitive deficits, often associated with hallucinations and fluctuating levels of consciousness), especially in older people. Patients (and their families) should be warned that people who experience these adverse effects should not drive.

Examples of drugs with possible anticholinergic effects include antidepressants, antipsychotics, antihistamines, antipruritics, antiparkinsonian agents, antispasmodics, and anti-emetics.

Impairment may result from episodic or regular use of so-called designer drugs and herbal substances; hence, patients should be cautioned about driving when using any of these products for psychoactive effect or for sleep.

All medications for Parkinson disease can trigger excessive daytime somnolence, which can be quantified by the Epworth Sleepiness Scale (http://epworthsleepinessscale.com/about-the-ess/). This problem is particularly evident with the newer dopamine agonists (ropinirole and pramipexole).

Patients requiring these medications must be cautioned about this risk and advised not to drive
if they experience daytime drowsiness or any episodes of falling asleep without warning or in unusual settings (e.g., during a conversation or a meal). Medication adjustment or treatment of an associated sleep disorder may allow safe return to driving after a period of observation.

Cannabis is a psychoactive substance containing the specific addictive ingredient delta-9- tetrahydrocannabinol (THC). THC affects the reward circuitry in the brain and results in cognitive and affective impairment that can affect driving ability, specifically by causing disturbances in perception and in reaction to the environment. Pharmaceutical products such as dronabinol, nabilone, and nabiximols (a combination of THC and a non-psychoactive substance in the cannabis plant, cannabidiol [CBD]) are available. Although cannabis is a legal substance in Canada now, it is considered harmful from a physiological perspective.

Following a court decision, Health Canada was required to render cannabis available for individuals who wished to use it for medical purposes (which occurred through the Marihuana Medical Access Regulations, now repealed). However, medical marijuana is not an approved drug, and its efficacy remains questionable to both scientists and clinicians. As the College of Family Physicians of Canada (CFPC) position paper on this subject states,

Smoke is a hazardous delivery system, because (a) psychoactive ingredients reach the CNS [central nervous system] much faster than other routes, causing intoxication; and (b) smoke contains hundreds of chemicals that are potentially carcinogenic or harmful to the heart or other organs. Similarly, there is no evidence for the safety or efficacy of oral ingestion of herbal cannabis (CFPC, 2013).

There is a direct dose–response relationship between blood THC concentration and impaired driving ability due to impairment in judgment, motor coordination, and reaction time (Ramaekers et al., 2004). No “low-risk” level of use has been established, and the dose is difficult to determine when cannabis is smoked.

Although the acute or chronic level of THC intoxication is difficult to gauge, it is notable that, after alcohol, cannabis is the substance most frequently found in drivers involved in fatal crashes (https://www.ccsa.ca/impaired-driving). A meta-analysis of multiple studies estimated that the risk of being involved in a crash doubled after cannabis use (Hartman and Huestis, 2013). Another study noted that higher levels of THC in the blood of collision-involved drivers was associated with a likelihood of being responsible for the crash three to seven times greater than for drivers who had not used drugs or alcohol (Brady and Li, 2014). The risk associated with cannabis in combination with alcohol has also been found to be greater than for either drug alone (Lenné et al., 2010). Increased crashes, injuries, and deaths are to be expected as rates of driving under the influence of cannabis increase with increased availability and use by drivers following legislative changes in various jurisdictions.

Statistics published by Health Canada, derived from the medical marijuana program before the recent legislative changes, showed that the average user was consuming 2.1 g of dried cannabis per day. This equates to about five joints of cannabis per day, since the typical joint contains between 500 and 750 mg of dried cannabis (Health Canada, 2019). The CFPC position paper recommends that patients refrain from driving for 5 hours after smoking a joint (CFPC, 2013). Consequently, the average user under the Health Canada program should not drive on any day when they have consumed the average amount of cannabis or for at least 5 hours after a single joint. Licensing agencies in Canada have encountered drivers consuming, according to their physicians’ reports, up to 30 g per day (45–60 joints) while supposedly remaining fit to drive. This is a dangerous situation, as physicians may be held liable for any mishaps that happen in the context of a dosage endorsed by the physician.

Physicians need to evaluate patients and discuss very carefully their use of marijuana with respect to the risk related to impaired driving. Recommendations to refrain from driving for a period after using cannabis must be provided in this discussion. It should also be emphasized that driving under the influence of cannabis, no matter the justification for its use, is illegal under the Criminal Code and subject to sanctions.

Recent public policy changes in various jurisdictions in the United States and the legalization of cannabis in Canada have increased availability, which (much like the situation for alcohol availability) is expected to increase impaired driving (Canadian Centre on Substance Use and Addiction, 2020). According to Statistics Canada survey data (Statistics Canada, 2020), there was a small increase, from 14% to 17%, in the proportion of Canadians reporting use of cannabis in the prior 3 months between 2018 (before legalization) and 2019. US data extrapolated to the Canadian context have been projected to result in as many as 308 additional driving fatalities annually (Windle et al., 2021).

Consideration should be given to informing the appropriate licensing authority if the medical condition for which the cannabis is being taken or the use of medical marijuana could interfere with a patient’s ability to drive safely, or if addiction involving cannabis is present.