Delhi police have arrested a teenager who posed as a doctor for at least five months at one of India’s most prestigious medical institutions.

Adnan Khurram, 19, was friends with doctors and medical students at the All India Institute of Medical Sciences (AIIMS) in Delhi, where he was active in hospital politics and posted pictures of himself to his Instagram page – @Dr.Adnan_Khurram – wearing a stethoscope and face mask.

Police, however, say Khurram was not enrolled in any medical training, had no qualifications and was receiving no salary from the hospital. He was arrested at the weekend after coming into the hospital to take part in a staff marathon.

Doctors said they had first become suspicious because Khurram, from Bihar state, seemed to have more free time than most junior doctors, who typically work shifts of up to 20 hours.

“We were surprised that he could make it to all the events at AIIMS and even those organised by other medical organisations,” Dr Harjit Singh Bhatti, the president of the hospital’s resident doctor’s association, told the Hindustan Times.

“He used to hang out near the coffee shop or the doctors’ hostel every evening. We started wondering how he had so much time.”

Police said Khurram had given different reasons for the fraud, including that he was trying to help an ill family member get faster treatment and that he hoped to be a doctor one day and in the meantime simply enjoyed pretending.

“He would roam around wearing the lab coat and stethoscope all the time,” Bhatti said. “We found out that he had made different claims to different doctors. To some, he would claim that he was a junior resident doctor, while to junior resident doctors, he would introduce himself as an undergraduate medical student. He had even made his way to the WhatsApp groups.”

There are about 2,000 doctors in the institution with new intakes twice a year, so keeping track was difficult, Bhatti added.

Khurram has been charged with impersonation and forgery.

Medical skills shortages across India have encouraged the proliferation of “quack” doctors. A 2016 study by the World Health Organization found that less than 20% of purported doctors in rural India had any medical qualifications.

The study found there were about 36 qualified doctors for every 100,000 Indians, compared with 130 in China and 2,710 in the UK.