All of the 51 websites selling products containing the potentially lethal industrial chemical 2,4-Dinitrophenol (DNP) are registered outside the UK, according to the Scottish Government.
After working with domain name registrars, 21 websites have been closed or no longer sell DNP and the FSA, a UK Government department, has now contacted registrars to request that a further 24 sites are shut down.
However, the NHS has warned that DNP can still be bought online easily.
An NHS Choices circular warning talks about the bid to stop internet sales, saying: "Many websites that offer DNP for sale are based in foreign countries meaning that cutting supply of the drug is difficult. Despite the best efforts of the UK authorities, people determined to buy DNP can do so with ease.
"Because it is so easy to access supplies, there is only so much the authorities can do to protect you. It takes only a few seconds to find sites selling DNP on the internet. Some of these sites offer such illegal products alongside legitimate weight loss drugs, which adds to the potential for confusion."
Two health officers in Scotland issued nationwide warnings in August about DNP being found in slimming aids. The substance is banned for human consumption but is available as it is also used as a pesticide. It has been linked with more than 60 deaths worldwide.
The warnings from Sir Harry Burns, Scotland's chief medical officer, and Bill Scott, the chief pharmaceutical officer, raised concerns DNP had been found on products labelled as 'fat burners'.
The Scottish Government said then it would work with the FSA, Health Protection Scotland, NHS Boards and councils to raise awareness over the dangers related to the chemical and to remove them from the market.
A Scottish Government spokesman said it continued to work with the FSA and others to advise GPs, councils and others of the dangers of DNP and how to identify potential poisoning cases.
He said: "The FSA has targeted gyms and sporting associations to educate members about the dangers of DNP. At the request of the FSA, local authorities recently visited gyms, seized suspicious products and sampled them for DNP. However, to date, no positive samples for DNP have been found.
"The FSA is working closely with the National Crime Agency (NCA) to shut down sites but the majority are run from outside the UK. In these cases, the FSA passes this information to their international counterparts to take action.
"In Scotland, individuals illegally selling these products face prosecution. Further work is being done with Defra to review the controls on the supply and distribution."
There were 16 cases of DNP poisoning in the UK last year, with three suspected in Scotland.
Among the deaths last year, in February Sarmad Alladin, 18, a student from Surrey, died after taking the pills.
In September 2012, Sarah Houston, 23, died in Leeds where she was a student after secretly taking DNP in the form of capsules. She had been seeking help for bulimia and at an inquest into her death in Wakefield, local coroner David Hinchliff said DNP was "entirely" responsible for Miss Houston's death.
Miss Houston had taken DNP along with a prescription anti-depressant. The combination of the two drugs, which both boost the metabolism, may have proved fatal, the inquest heard.
DNP is sold mostly over the internet under a number of different names. The manufactured drug is yellow and odourless and was previously used as a herbicide and fungicide.
It was launched as a slimming aid in the US in the 1930s but then banned in 1938 due to its side-effects.