If you're feeling sleep-deprived you may be tempted to take sleeping aids to help you nod off.
But experts are warning that one sleeping tablet, that is available to buy online, can become "addictive if used for more than a few weeks".
The medication, called zopiclone, should only be taken following a prescription from a doctor.
But writing in the British Medical Journal, a group of doctors warn it is easy to find the drug online. They also raise concerns about the potential for the drug to be misused.
"Z drugs" - the name given to a group of drugs used to ease symptoms of insomnia - can be prescribed by a doctor for short periods of time. Zopiclone is one of these drugs.
According to the study authors, the use of zopiclone appears to be increasing. In 2013-2014, zopiclone was the sixth most common drug among enquiries to the UK National Poisons Information Service’s online database, TOXBASE.
Z drugs, such as zopiclone, are not normally prescribed for more than two to four weeks because of concerns about dependence and addiction.
To investigate the availability of zopiclone, the researchers identified 37 websites selling the tablets in quantities of up to 2000.
A total of 35 of these websites also sold similar drugs and 15 offered bulk purchase discounts.
Most (24) provided information and warnings about dosage, but 22 clearly stated that no prescription was necessary for purchase, while 14 made no mention of this at all.
Only one website stated that a prescription was needed.
"Not only do these websites bypass necessary oversight required for supply, they provide access for vulnerable people who may buy it for self-poisoning, suicide, or misuse," the study authors argue.
"Regulatory authorities need to ensure that appropriate measures are taken so that these websites comply with medicines and other regulations for the supply of this prescription only medicine."
Teva is one of the companies that market a version of zopiclone in the UK.
A Teva spokesperson tells HuffPost UK Lifestyle: "That patients are receiving quality medicines legitimately is of prime importance to us and we invest a lot of time and effort in ensuring our supply chain retains its integrity.
"Prescription only medicines, like zopiclone, should only be taken by patients who have been prescribed them by a doctor or other appropriate healthcare professional, according to the individual patient’s needs."
Commenting on the drug, Dr Helen Webberley, the dedicated GP for Oxford Online Pharmacy, says zopiclone "initially works well at inducing sleep, which is not the same as naturally occurring sleep".
However, she says the medication has two key problems.
"Firstly, it is highly addictive, so once you have used it for a short time your body relies on it and finds it very hard to get to sleep without it," she says.
"Secondly, as time goes on your body builds up a tolerance to the medication so it stops working as well, and your body craves higher and stronger doses."
She adds that is it imperative that drugs such as zopiclone are carefully monitored by doctors and should be used for very short bursts in exceptional circumstances.
"I have spent a lot of time in my position addressing patient addiction to medication like this, and I feel strongly that it should be a Prescription Only Medication," she says.
"Prescription Only Medication is only available with a prescription for a very good reason. The prescription requires the involvement of a GP enabling them to diagnose the condition and identify the best medication to treat the individual patient.
"If you are being offered Prescription Only Medication without a prescription the message is simple: do not buy it - it is illegal and there is a strong chance that what you are buying will be fake."
SEE ALSO:
10 Things to Try Before You Go to Your Doctor for Sleeping Tablets
Sleeping Pill Side Effects: May Be Linked To Increased Heart Attack Risk Say Scientists
