The response was overwhelming. The doctor was inundated with questions about drug interactions, risks of use in people with specific medical conditions, dosage levels, adverse effects and toxicity. Users asked about enhancing sexual experiences with ecstasy, alleviating cancer-related pain with cannabis and how to store psychedelics to ensure their longevity. Caudevilla, who lives in Spain, answered them all. Well, nearly all. "I was asked once about the dose or combination of benzodiazepines to knock out a person without his knowledge and I would not answer a question like this," he says. "Questions about drug dosage for suicide are also delicate. I think people should have rights to decide about their own death in some circumstances but I do not think an internet forum is an adequate tool for this." His advice thread on Silk Road has had more than 64,000 visits and he has answered 750 questions in it. He is also active on The Hub, a centralised discussion forum for all the dark markets (that cannot be accessed on the regular internet) and answers questions by private message. "The doc is a pillar of the community" wrote one user, TheChain, last month. "If I went to my doc with some of the questions he's answered I would get the same old 'drugs are bad' response even though I was going to do it without his consent."

This refusal to pass judgment has made Caudevilla, who lives in Spain, one of the most respected voices of Silk Road. When one member said they were thinking about trying dissociatives (a class of hallucinogen, which distort perceptions of sight and sound and produce feelings of detachment or dissociation), the doctor went through the effects and dosages of each, and provided his recommendation and the problems associated with it Caudevilla makes no apologies for what some would consider his radical approach. In this environment he does not bother with attempting to dissuade people from using drugs, unless their use might harm another, for example by breast feeding or driving. But by the time someone has become a member of Silk Road, he thinks they have gone far beyond being merely curious about drug consumption. "So it makes no sense to use general prevention messages," he says. Global drug policy is based on a repressive legal system that has proven to be ineffective and has caused many more problems than the ones its supposed to solve,’’ he says. ‘‘ I think dark web marketplaces will be an important factor that will force a change in drug policy in a near future.’ Fernando Caudevilla His harm reduction work is tailored to his audience. "If I talked to a classroom of 13 years-old schoolboys I would not explain to them how to use cocaine, of course."

Caudevilla lives in Madrid with his fiancee and works in a family clinic by day "as a normal doctor," he says. But his main interest is in drugs and he works with Energy Control, a Spanish NGO, providing drug testing at parties and raves. "I dedicate all my free time in different projects around drugs and harm reduction. It just has been a field that interests me". Caudevilla does not see the dark web drug markets as pure evil. "Global drug policy is based on a repressive legal system that has proven to be ineffective and has caused many more problems than the ones its supposed to solve," he says. " I think dark web marketplaces will be an important factor that will force a change in drug policy in a near future." There have been global law enforcement efforts to close down this new avenue of drug dealing. The FBI arrested Texan Ross Ulbricht in October last year and he faces trial in November, accused of being the mastermind behind Silk Road. Many of the site's top vendors from have been arrested in various countries. Brisbane man Peter Nash was extradited to the United States to face conspiracy charges as an alleged moderator of the discussion forum where Caudevilla dispenses his advice.

But the seizure of the original Silk Road did little to slow down the online drug trade. Illegal drugs are now available through more than 20 dark markets and Silk Road is no longer the market leader. New websites Evolution and Agora, which have more sophisticated infrastructure and more customers, have around 60 per cent of the overall listings of illicit goods between them. Silk Road is in third place with around 20 per cent. Yet Caudevilla points out that the dark web's self-regulating markets often act more ethically than their legal counterparts. When Silk Road was hacked earlier this year, the administrators repaid the money stolen from members' accounts. "Dark web drug market administrators are more trustworthy than many politicians or Wall Street businessmen," he says. Caudevilla's focus is firmly on reducing the potential for harm from illegal drug use. His dosage guidelines for novices are conservative and he arms them with facts to help them make an informed choice. One Silk Road member, Trust In Us, wrote that his girlfriend had overdosed and contacted Caudevilla who happened to be online at the time: "He gave her the directions she followed and lived."

Another claimed Caudevilla's concern that his heroin withdrawal symptoms were atypical drove him to seek medical assistance. He discovered he had early-stage leukaemia. Questions come from all over the world: "A person from United States explained that he couldn't ask to his doctor about drugs because his medical insurance was provided by his employer and he was afraid of them finding out," Caudevilla says. "And some people from Russia or Asian countries buy prescription opiates from Silk Road and ask for advice about how to use them to detoxify from heroin, as in their countries there are no programs available." Caudevilla provides a discreet link to a bitcoin wallet to which he can accept personal donations. Caudevilla has a qualified respect for the creator of the website. "[The founder] was a very charismatic character and has become a hero or a martyr for his community. I think his idea of Silk Road is amazing but I do not agree with some of his ultra-anarchist libertarian discourse," he says of Dread Pirate Roberts who followed Agorism, a form of revolutionary market anarchism. "He thinks people should have the same right to use cocaine as to have a rifle in their home. I think there is a huge difference between potential harms of self-use of cocaine and potential harms of an assault rifle".

Few people would be in a better position to judge Caudevilla's work than Dr Monica Barratt who has studied the dark markets in her role at Australia's National Drug and Alcohol Research Centre. She co-ordinated the inclusion of questions about Silk Road into the Global Drug Survey last year. "DoctorX provides a unique service by actually going to where people who need assistance actually are: you could call it digital outreach," she says. "Rather than expecting people to flock to a website or download a new app, outreach workers should consider finding where people already congregate (whether that be a physical place, or a dark net market community forum), then offering assistance there and then." Dr Alex Wodak, president of the Australian Drug Reform Association, has read some of Caudevilla's advice. "From what I have read, he seems to take this work seriously," he says. "He is patient with people who seem to have little time for science. I think he should more often point out that abstinence is always an option – which is the case – even if he knows that his readers seem to have made their minds up about taking the drugs they discuss with him". Dr David Caldicott, an emergency consultant at the Calvary Hospital in Canberra, provides a similar service to Caudevilla on the regular internet. He posts in the popular drug safety Bluelight forums under the name Dr Platypus. His focus is also firmly on harm reduction, but his approach is more conservative than Caudevilla's. "We shouldn't be encouraging drug use but we need to accept that drug use occurs and we need to explain to people how not to get hurt,' says Caldicott. "[Caudevilla's advice is] a little a bit more consumer-oriented than my own approach."

Both Caudevilla and Caldicott are advocates for drug reform. But they have different opinions on how that reform should be achieved. "I am absolutely in favour of total legalisation of all prohibited drugs," Caudevilla says. "This does not necessarily mean that you can buy methamphetamine as easily as tomatoes, or that heroin should be advertised in glamorous ways like cars or perfumes. "Different kinds of substances need different regulations. I think that if we have spent 90 years in an ineffective and harmful moral-based system, we can spend the next 90 testing solutions based on scientific evidence." Caldicott doesn't agree with this no-holds-barred approach. "Total legalisation of all of the drugs for everybody doesn't make sense at all," he says. "I'm in favour of a graduated response to every compound that can be consumed with decisions made from evidence based on their dangers and hazards… If a drug poisons a quarter of people who consume it or their behaviour causes people harm, then definitely it should be illegal." While many are horrified that technology has brought drug dealers into the loungeroom, others claim that it is a safer alternative to scoring in the traditional way – from a friend, acquaintance or local dealer.

The eBay-like feedback system and desire of competing vendors for repeat business provides some semblance of market-driven quality control. Anonymity of internet shopping eradicates any threat of violence. "The benefits are many for buyers and thats one of the reasons for success of these markets," says Caudevilla. Caldicott is not convinced. "I'm a little sceptical about arguments that say dark web product is safer. Were we to prove that we would have to acquire numerous products on the dark web and actually test and analyse them." Interestingly, the FBI analysed over 100 purchases during 2012/13 during its operation to bring down Silk Road and revealed: "Samples of these purchases have been laboratory-tested and have typically shown high purity levels of the drug the item was advertised to be on Silk Road". Alex Wodak says: "I think online distribution is less harmful than street sales because the risk of violence is less. But online sales are still not regulated and that troubles me." The number of illicit drugs listings on the dark web has more than doubled since the arrest of Ulbricht. "Once a technological advance is established there is no way back," Caudevilla says. And DoctorX will be there providing their customers with drug safety advice.

Eileen Ormsby is author of Silk Road being released by Pan MacMillan on November 1.