Police confirmed to the Phuket News that the dead woman was a New Zealand national.
According to the outlet, Karon police said several prescription medications, including antidepressants, as well as two cocktail glasses and three open bottles of alcoholic beverages, were found in her room near Kata Beach.
A preliminary forensic report suggested that a reaction between the alcohol and medication possibly led to respiratory failure, officers said.
A 49-year-old German man, who was a friend of the woman, alerted hotel staff after he found she was not breathing.
The staff then alerted police, who immediately sent officers and a doctor to the scene.
The man told police the two of them were out drinking before returning to the hotel room at 1am (local time).
The German was awoken by the cleaning staff around noon when he realised she was not breathing and called hotel staff.
CCTV footage and hotel entry records confirmed his story, police said.
Bar staff where the pair were drinking reportedly told investigators that no unusual incidents or disputes occurred before they left.
When police took the German in for questioning, he tested negative in both a drug test and a physical examination, it has been reported.
According to the Phuket News, forensic investigators estimated the woman had been dead for 8-10 hours before her body was found.
Police confirmed to local media that the case is still under investigation.
The woman’s body was taken to hospital to determine the cause of death.