Use of higher than recommended doses of dextromethorphan has the ability to produce any of the following symptoms:

Short Term

• Nausea and/or vomiting
• Diarrhea (infamously known as the "roboshits")
• Urinary retention
• Allergic reactions and itching (commonly referred to as roboitch)
• Hot flashes and increase in body temperature
• Diziness
• Increased perspiration
• Hypertension
• Increased heart rate
• Drowsiness or insomnia
• Dilated pupils
• Blurred vision and/or nystagmus
• Bad trips and panic attacks
• Psychotic breaks/mania
• Temporary cognitive impairment
  

Long Term

• Brominism
• Urinary retention (seems to be more severe with regular use)
• Liver, Kidney, and Pancreas Damage (from consumption of sugars and thickeners in syrup. There is no evidence that DXM alone can cause this)
• Weight loss
• Tolerance
• Psychological addiction and depression upon quitting
• Psychotic breaks/mania (more common with frequent use)
• Temporary cognitive impairment (can last longer after quitting with more frequent use)
• Irreversible brain damage (this is often confused with long term temporary impairment)
  

Brain damage is fairly rare occurring in less than 1% of regular users (who all used DXM very frequently). Keep in mind that this damage may not necessarily be permanent as Olney's lesions are not proven to occur in humans However, long term impairment can take a while to resolve itself. Some heavy users of DXM report full recovery talking several months to years after quitting.

If you suspect damage has occurred, stop using DXM immediately. Consider starting an exercise program, a hobby to keep your mind active and supplements such as piracetam and phosphatidylserine to alleviate NMDA receptor down-regulation.[1] Keep in mind that depression (a potential side effect of quitting DXM suddenly) can also cause neurodegeneration and may exacerbate the impairment.