In the US thriller film Limitless, a struggling writer whose career is heading nowhere pops a mysterious drug, which unlocks his untapped cognitive abilities, turning him into a financial genius and literary whiz.

Hoping to gain a similar edge, some young people here, from undergraduates to secondary school students, are using "smart drugs" to help with last-minute cramming. Students as young as 16 are buying and taking drugs such as modafinil to get better exam grades.

And they are finding it easier to obtain such drugs. Some students share them with their peers. It is also possible to get the drugs on online platforms, such as Carousell, and messaging services like Telegram.

Some sellers offer free samples, delivery services and even back-to-school promotions. A blister pack of 10 pills may cost between $15 and $35, depending on the potency.

Such drugs - used to treat disorders such as excessive daytime sleepiness or conditions like attention-deficit hyperactivity disorder (ADHD) - are touted to boost concentration, decision-making and memory power in healthy people.

One website selling modafinil - used to treat people with narcolepsy and other sleep disorders - targets Singapore students "who are mercilessly subjected to an intense education curriculum". It added that, with modafinil, students can "study more in less time". The seller said his youngest buyer is in Secondary 4.

Pills containing modafinil are marketed under names such as Modalert and Modvigil. One pill, which costs about $1.50, can keep a person awake for several hours. Other cognitive enhancers include methylphenidate, marketed under formulations such as Ritalin and used to treat those with ADHD and narcolepsy.

Observers believe more students are misusing these "smart drugs". Dr Lim Boon Leng, a psychiatrist in private practice, said more students who do not have clinical conditions that require treatment are approaching doctors for the drugs. He said it can be dangerous to buy these drugs off the streets or over the Internet and to use them without supervision.

Some studies have shown that these cognitive enhancers can produce mental gains in normal people, but they can also cause side effects such as heart problems, severe rashes, headaches, irritability, difficulty in breathing and insomnia.

Psychologist Daniel Koh from Insights Mind Centre said the lack of rest may slow down reaction time, affect moods and the immune system, or result in a breakdown. "The body and mind cannot be constantly stimulated and not allowed to relax," he said.

Dr Thomas Lee of The Resilienz Clinic has encountered a handful of students over the years who admitted storing up methylphenidate for exams. Methylphenidate is listed as a controlled substance under the Misuse of Drugs Act.

Dr Lee said: "They pretended that they were taking the medication daily but secretly accumulated the supply to take during exams and tests. Sometimes, they shared them with or supplied them to their friends."

In a joint reply on methylphenidate, the Health Sciences Authority (HSA) and the Central Narcotics Bureau said medication containing the substance needs to be registered with the HSA, and can be prescribed only by a registered medical practitioner.

"Prior approval from HSA is required for each import of therapeutic products that contain controlled drugs," they added.

Anyone who imports or supplies, without prior approval, a health product with methylphenidate may be jailed for up to two years and/or fined up to $50,000.

Modafinil, while not a controlled substance, is not registered for use as a therapeutic product here, the HSA noted.It said that, if convicted of unauthorised sales, a person can be fined up to $50,000 and/or jailed up to two years.

Jalan Besar GRC MP Denise Phua, who heads the Government Parliamentary Committee for Education, urged students to refrain from resorting to these drugs to perform better in their studies.

"It is smarter to stick to natural strategies such as having enough sleep, healthy food, lots of physical exercise and adopting good study skills - strategies that are all tested and backed by research," she said.