• Feeling drowsy or can’t think straight

• Feeling dizzy, light-headed or faint

• Less coordinated, shaking or feel shaky

• Feeling edgy, angry aggressive or unstable

• Feeling nauseous or otherwise unwell

Jane Stutts study showed that people in sleep-related crashes were more than four times as likely to be taking medication as others. 

Apart from drowsiness drugs may raise or lower blood sugar or blood pressure, making you feel faint and some affect vision eg accutane. Extreme drug reactions of concern are seizures or hallucinations.

Drugs to possibly exercise caution with if you drive

Medication side effects will vary from person to person, depending on age and individual health factors. And even on variables like how well hydrated a person is.

Prescription drugs that may have sedating effects include narcotic pain killers, anti-hypertensives (for high blood pressure), older antidepressants, cholesterol-lowering drugs, sleep medications, anticholinergics, anti-anxiety and sleeping pills (tranquilizers) and some older antipsychotic drugs.

Anti-psychotics also known as major tranquilisers likely improve driving as compared to personal baselines of untreated users - but some will sedate in the first week or with dose adjustments 

Common remedies on the drugstore's shelf that can contribute to hazardous driving include pain pills (including nonsteroidal anti-inflammatory drugs or NSAIDs), diet pills, sleeping pills, cough syrups, cold formulas and allergy medications. Also natural calming drugs like kava calmThey can... but generally don't if properly used.

Fortunately studies on balance show no significantly heightened crash risk even for users of the medicines which are classed as high risk drugs. This typically holds true so long as medicines are taken as directed - in the right dose, and at the right times with some common sense used about driving if feeling under the weather. 

It is really only when risk drugs are used in very specific ways in order to achieve highs that the users, or rather deliberate abusers land in tolls or cause others grief.

BENZODIAZEPINES (valium, halcion, rivotril etc)                                                                       

If starting on these medicines avoid driving for 2 weeks if not adjusted. And thereafter for 5-10 hours after a dose if feeling dopey. Some people may be affected similarly to being at the alcohol limit while on this sort of medicine but most adjust and will be safe to drive. Serious crash risk is only proven to greatly increase if alcohol or other intoxicants are combined with benzos used as per prescription. 

It may pay to avoid early and night driving, stick to familiar routes, prefer short journeys in low speed zones. Take advantage of home deliveries and use other transport or ask people to drive you if you’ve reason to believe you're impaired.  

If taking a benzodiazepam drug eg Clonazepam (Rivotril), Triazolam (Halcion), Nitrazepam (Mogodon) etc for anxiety or sleep problems, skipping your dose/s with a worsening of the treated condition could also impair driving. Take special care to monitor your own driving fitness after restarting or increasing benzos. 

Decongestants & cough syrups

In winter many people take decongestants. Possible side effects of these OTC drugs include raised blood pressure, anxiety, restlessness, dizziness and hallucinations ‹all of which can interfere with safe handling of a motor vehicle. So can the depressive effect of alcohol-containing cough syrups.

Antihistamines

These block an allergic reaction, they may also slow down reaction time and impair alertness and coordination. If you are an allergy (or cold) sufferer who takes this medication regularly, ask your doctor or chemist for a non-sedating antihistamine.

Alternatives that won't make you drowsy may be available for other medications, too. Sedating versions (cheaper) are relatively low risk in traffic but can greatly worsen effects of drugs such as benzos or alcohol. 

University of Iowa researchers who tested 40 allergy sufferers in a driving stimulator found that the antihistamine diphenhydramine ( in many allergy and cold medications) significantly impaired a driver's ability to "track" or follow, steer, and maintain the correct lane. 

The study showed that diphenhydramine has greater impact on driving performance than alcohol does.  Medical professor John M. Weiler story said this was surprising. Fortunately the lab studies are not translating to high crash rates for allergy patients or those using anti-histamines for travel sickness.

OTC weight reduction aids

OTC weight-reduction drugs, which some people take to combat fatigue, may over- stimulate, affecting judgment, reflexes, cognitive skills and coordination. They can also cause dangerous disorientation.

Don't stimulants enhance driving performance?

Although stimulating substances, such as caffeine, may briefly improve some aspects of performance, such as alertness, they may impair discernment skills essential to such maneuvers as sudden stops. Vision is distorted.

Old generation sedating anti-depressants (tricyclics like amitriptyline) – can slightly impair driving due to sleepiness and increased brain theta waves (cause weaving) in the first week of use. No increased crash risk exists once users have tolerance or among the using population overall. It's simply wise to be aware some antidepressants could be a problem early days /  in combination with other drowsy drugs.  

SSRI Antidepressants

Like "prozac" are not sedating. Some people are however better to take anti depressants that are, as a means to get better sleep in early treatment days.

The right drug, best dosing times and your travel needs are best discussed with your prescriber. Major studies haveshown no over representation of anti depressant users in deceased drivers. 

PHYSICAL LIMITATIONS 

Conditions that can impair driving include heart conditions, Parkinson’s disease, epilepsy, diabetes, chronic pain, injuries, Alzheimer’s or other forms of dementia, and arthritis or other joint problems. People with such conditions should discuss with a doctor how their driving can be impacted, and be aware of their limitations.

Maintaining an exercise program is also important. People often think of driving just as a mental task, but it’s quite physical.

“People who exercise will be better able to control wheel and pedals, especially for older people. Having good leg muscle and arm muscle control is important,” she says.