The Care Quality Commission has warned people to be careful when considering buying medications on the internet.
The watchdog's inspection of some online prescription services in England revealed significant concerns over patient safety.
The regulator said some well-run providers offered a convenient and effective form of treatment.
But some sites put patients at risk by too often selling medications too quickly without enough checks.
The CQC has published reports of two urgent inspections.
One followed a BBC 5 live Investigates programme highlighting concerns about Treated.com, run by HR Healthcare Ltd, in October 2016.
The CQC inspected as a direct result and suspended its registration in December 2016.
Prof Steve Field, Care Quality Commission chief inspector of general practice, told the BBC: "Your investigation was really important.
"We started to look at these remote-prescribing doctors and what your investigation did was help us prioritise.
"We brought forward a number of our inspections so we have now looked at 11 providers, two of which we have published today.
"We were shocked at what we found".
The second provider, MD Direct Ltd, voluntarily cancelled its registration after the inspection in December last year.
The CQC found both providers demonstrated significant safety and organisational risk to patients, with widespread failing to provide safe care.
The main concerns identified in both included little-to-no verification of patients' identity, inadequate prescribing and no assurances the clinicians working behind the sites had the relevant skills or qualifications for the role they were performing.
Riaz Vali, responsible for Treated.com, told the BBC it was making improvements to its processes and systems.Image copyright SPL
Charlie Massey, chief executive and registrar of the General Medical Council, which regulates doctors, said he welcomed innovation and supported remote prescribing when it followed the GMC's guidance, but said it was clear not all practitioners were following the guidance.
The GMC was not able to comment further on individual investigations that followed the CQC's inspections.
The CQC has released new guidance on what patients should expect from good service, such as being asked to upload photo ID, taking a detailed history of medical problems and asking permission to contact their GP.
And it has published its first guidance to online providers about how it inspects services in England.