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Hello,
Dear customer
Regarding what you mentioned,

What are the most common causes of painless red eye ?

• This may be due to conjunctivitis which could be due to either bacterial or viral infection.
• This may be due to history of trauma and burst blood vessels or it may result from persistent coughing , straining or as a side effect of certain medication such as aspirin.

What is the exact cause of this case ?

Without doing a direct clinical evaluation of the case we cannot determine the exact cause of this case .

What should you do in this case ?

 we advise you to keep the affected eye clean and you have to visit the ophthalmologist doctor so  that the doctor could detect the exact cause then he will treat it accordingly.
  Stay well.   
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