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You should seek medical attention immediately after a whiplash injury. A complicating factor is that the pain and disability of whiplash can take a day or even a week to fully manifest, but see your family physician soon after any major trauma to your head and neck in order to rule out life-threatening injuries (fractures, dislocations, internal bleeding).  Your doctor will likely take an x-ray of your neck (cervical spine) to rule out obvious fractures or dislocations of the vertebrae or facet joints. If you're in severe pain and having difficulty holding your head up, then you may be given a foam neck support collar for short-term use. However, research has shown that wearing a stiff cervical collar for much more than a few days can cause neck muscles to atrophy (weaken) and prolong pain. Your family doctor is not a musculoskeletal specialist, so if your neck feels severely injured then you may need to see a specialist for a second opinion. A specialist such as an orthopedist may take more x-rays, an MRI or a CT scan of your neck / head to better understand and diagnose your whiplash injury.  In addition to bone injuries, MRI can detect soft tissue injuries, such as spinal cord damage, herniated disks or torn ligaments.  Before starting physical therapy your doctor(s) need to determine if your neck is structurally intact, physiologically stable and able to safely withstand stretching and exercising. Sharp or burning pain combined with grinding sounds with movements, shooting pains into your arms and severe dizziness are signs suggesting neck instability. Before you start to stretch and exercise your neck, you need to reduce the inflammation and pain. Your doctor may recommend non-steroidal anti-inflammatories (NSAIDs) such as ibuprofen or naproxen for short-term use, although if your pain is severe, you might get a prescription for something stronger — typically an opioid, such as oxycodone.  Alternatively, you can try over-the-counter analgesics such as acetaminophen (Tylenol) or muscle relaxants (such as cyclobenzaprine) for your neck pain, but never take them concurrently with NSAIDs. Keep in mind that these medications can be hard on your stomach, kidneys and liver, so it's best not to use them for more than 2 weeks at a stretch. The application of ice is an effective treatment for essentially all acute musculoskeletal injuries, including neck pain. Cold therapy should be applied to the most tender part of your neck for 15-20 minutes every 2-3 hours in order to reduce the swelling and pain. Always wrap ice or frozen gel packs in a thin towel in order to prevent frostbite on your skin.
See your family doctor. Make sure your neck is stable. Control the pain and inflammation.