Motion sickness in dogs while travelling in vehicles is a common problem. This is especially common in younger dogs. Motion sickness is caused by motion and is due to unfamiliar accelerations, and intersensory conflict between visual and vestibular sensors1. Motion sickness results in nausea, vomiting, lack of appetite, diarrhea and apprehension for several hours after the cause of the issue.
The vestibular organs
The inner ear contains your dog's vestibular organs. The vestibular organs are made of three semi-circular canals and two otolith organs2. The semi-circular canals sense rotational acceleration. Otolith organs sense linear acceleration. When a conflict occurs between your dog's vestibular, vision, and proprioceptive motion sickness is likely to occur. The reasons behind this can be highly complex however motion sickness predominantly affects puppies. The development of your puppy's ears continues to take place until they are approximately six months of age. This can result in a conflict between your dog's vestibular system, proprioceptive, and visual systems causing motion sickness 3. Here is a good example of motion sickness "Imagine you are driving in a car over a slightly bumpy straight road, your balance organs and your proprioceptors sense that you are moving up and down, while your visual system may detect little movement.
Motion sickness is a very complex issue; a complete theory is not currently available that covers all of the problems. It can be affected by a lack of sleep, inner ear issues, and visual, and psychological issues. Studies have found that at some stage everybody will be affected by motion sickness when there is a conflict between your dog's vestibular system, visual system, and motion.
Visual system and motion sickness
Your dog's visual system also plays an important role in inducing motion sickness and this may arise due to a number of issues:
Binocular Vision Dysfunction: This is where your dog's eyes are misaligned and the brain struggles to send a clear image signal to the brain4.
Statismmus: Is a condition when one or both of your dog's eyes are misaligned. This can be either vertically or horizontally.
Vision impairment due to age: Vision impairment due to old age can also lead to motion sickness5.
Psychological causes.
Dogs can become anxious and nauseous while travelling in vehicles due to the unfamiliarity with driving and overwhelming stimulus. This increases the likelihood of experiencing motion sickness.
How do you tell if your dog is suffering from motion sickness?
Whining, barking, and pacing in the car
Vomiting
Lethargy
Diarrhea
Excessive lip licking
drooling
If you believe your dog is suffering from motion sickness, stop the car and let them sit outside in the fresh air. This may temporarily help them.
What can you do to help your dog who suffers from motion sickness?
Desensitisation: Take your dog on regular short trips before taking them on longer trips. This helps them get used to being in a vehicle and desensitize them to the visual and motion stimulus. Keep the music down low, and the temperature of the car cool. We recommend rewarding your dog with treats, taking them to places nearby they associate with fun, and ensuring the entire experience is positive. Do not tell them off if they seem agitated or nervous.
Car safety harness or seat: We recommend buying a comfortable dog seat and harness. This will give them their own personal space to lay down in comfort, keep them safe if you have to brake heavily, and will also help reduce dog hair on your car's carpet. 
Toys: Bring soft toys with your dog on car trips. You want to keep stress to a minimum and keep it fun.
Withholding food: Do not give your dog food within four hours of going for a long drive. We feel sick on a full stomach with lots of motion too.
Aromatherapy: Apply a few drops of chamomile or lavender onto the material on your dog's seat. Both of these are known for their calming abilities. Studies conducted on mice showed that the scent of conflict reduced stress and conflict in mice6. This is due to the calming smell of lavender and its effect on the parasympathetic nervous system.
Regular breaks: This is especially important if your dog is not used to car rides and you have to cover a long distance. Stop regularly, and take them for a short walk, or play. This will help to keep the experience fun for them.
If none of these tips seems to work we recommend taking your local vet. They will be able to supply Anti-Nausea and Anti-anxiety medication for your dog. Vets will only prescribe anti-anxiety medication for dogs that suffer from extreme stress.
References.
1) Brandt T, RB. The multisensory physiological and pathological vertigo syndromes. Ann Neurol. 1980 Mar;7(3):195-203. doi: 10.1002/ana.410070302. PMID: 6999976.
2) How does our sense of balance work? https://www.ncbi.nlm.nih.gov/books/NBK279394/
3). Motion sickness in dogs https://www.ncbi.nlm.nih.gov/books/NBK557588/
4) How does our sense of balance work? https://www.ncbi.nlm.nih.gov/books/NBK279394/
5) BVD symptoms and treatment https://www.optometrists.org/general-practice-optometry/guide-to-binocular-visual-dysfunction/bvd-symptoms-and-treatment/
6) Umezu T. Behavioral effects of plant-derived essential oils in the geller type conflict test in mice. Jpn J Pharmacol. 2000 Jun;83(2):150-3. doi: 10.1254/jjp.83.150. PMID: 10928328.