The sites are completely legal, and fall under the jurisdiction of a regulator called the Care Quality commission. Not surprisingly, the most in-demand drugs are for erectile dysfunction, sexually transmitted diseases and hair loss.
The websites don't handle serious medical problems or emergencies and don't deal in narcotics, painkillers or other drugs people could become addicted to.
"The British websites are definitely an exception, but they are the start of a trend we will soon see everywhere," said Dr. Steinar Pedersen, a founder and special adviser at the Norwegian Centre for Telemedicine.
"Opinions will change as people become more comfortable with technology," he said.
Pedersen didn't know of any countries beyond Britain where online medicine for patients who don't know the doctors and don't ever speak to them is legal. The European Commission does not monitor such medical websites, but is working on a paper addressing the legal issues of telemedicine.
In countries including Canada, Denmark, Norway, Germany, and France, doctors are only allowed to treat patients online if they have previously seen them in person.
In the United States, several companies offer online medicine, but patients must typically speak to a doctor on the telephone or set up a videoconference for a live, face-to-face chat.
The Canadian Medical Association said doctors who approve prescriptions for patients they haven't seen in person face losing their licenses.
But that's not the case in Britain, where rules are more lax.
The Care Quality Commission only oversees the work of doctors in private practice. Practitioners who work for the government are exempt from regulation altogether. If drugs are prescribed, the sites must be registered with a drug regulatory agency.
The websites typically charge for a doctor's consultation fee from about 8 pounds to more than 20 pounds ($13 to more than $30) that is not covered by the national health system. Some even ship a very small percentage of their drugs abroad; in Europe, it is legal for medication to cross borders as long as it is for personal use.
The commission monitors medical websites primarily to ensure the doctors running them are licensed.
"We inspect their offices if we get complaints, but otherwise they are monitored by self-assessment," said a spokesman for the commission, who did not want to be identified in line with government policy.
He said since the commission first began registering the websites several years ago, there have been few complaints. "They tend to treat conditions that are relatively straightforward," he said. If patients indicate any worrying symptoms, the websites tell them they aren't eligible for online treatment and need to see a doctor.
Dr. Patrick Cadigan, a cardiologist and spokesman for Britain's Royal College of Physicians, described Internet-based medicine as "second-best," and said it was particularly difficult to make a diagnosis without seeing a patient in person. "To lose personal contact with your patients means you lose clues about what may be wrong with them," he said.
Cadigan worried some patients may not understand the difficulties of being treated online.
He said some people might need some more in-person prodding, as opposed to the standard questionnaires employed by most websites, to correctly answer questions about other health conditions or medical treatment they were already on.
"If you don't get a thorough medical history from the patient, you could prescribe something that might have adverse effects," he said. "I am concerned these websites could be steering patients to treatments for a financial incentive rather than for their own good."
On the London-based website www.dr.thom.com, patients fill out forms to see if they are eligible for online treatment for things like impotence and birth control. Before getting mailed Viagra, for example, patients must fill in a questionnaire that covers their impotence, medical history, height, weight, and blood pressure.
About one in 11 patients doesn't qualify, in which case the site recommends they see a doctor in person. The site has at least 100,000 patients, including those from a popular pharmacy chain with whom they are linked.
Dr. Thom Van Every, the site's medical director, says patients give a medical history and are asked about other drugs they're on to avoid any dangerous interactions. On his website, patients with acne or genital warts, rashes or other abnormalities, can also upload a couple of pictures of the affected area. Doctors then diagnose the condition and if appropriate, send drugs in the mail.
"Most of the people who report this as strange are doctors," Van Every said. "People are so used to uploading pictures these days for things like Facebook that this is just not a big deal."
Dr. Lori Heim, president of the American Academy of Family Physicians, said examining pictures to make some diagnoses may be OK, but warned of potential problems such as doctors missing symptoms elsewhere on the body the patient hasn't photographed.
She said if patients had multiple symptoms or a condition that naturally required a physical exam, like listening to the heart, lungs or conducting joint exams, seeing a doctor virtually wouldn't work. She said doctors should be particularly careful about prescribing drugs like Viagra and worried about the web sites becoming prescription mills.
"The best thing is to have a relationship with your physician because you're more likely to get a better diagnosis," she said.
She said the online sites were useful for people who don't live close to a doctor, but said there were limits.
"If you have something that is potentially urgent, that's not the time to be messing around with the Web or email."
Heim predicted similar Web sites would pop up in the U.S. eventually.
"The technology ... expands services to patients who may not currently have access," she said.
Earlier this year, Van Every's site introduced an at-home cervical cancer test with a special tampon.
Peter Sasieni, an epidemiologist at Cancer Research U.K., said the charity did not support the wide availability of the test because there isn't enough data to prove it works. He said women who shouldn't be screened, like those under 25 might order the test online, get a false positive and then face further unnecessary exams and anxiety.
Doctors working online said patients must accept some responsibility for their own safety.
"There are always people who will do things that are not in their own interest," said Dr. Tony Steele, who runs the online service, Doctor Fox.
He said that for drugs like Viagra, patients are explicitly warned about the dangers of taking it if they are also on heart medicines with nitrates. Steele said that if patients order excessive amounts of certain drugs, doctors call the patient to find out what's going on.
Ceri Jones, who used an online doctor to prevent seasickness, said she wouldn't use the Internet for more complicated issues.
"If I had something like heart palpitations, I wouldn't go online," she said. "But if it's something simple and I've researched what I need, then seeing a doctor seems like a waste of time."