Will Nutland is not infected with HIV, yet he has been taking an Aids drug for just under a year. As a doctor, he says his primary motive is to act as "a guinea pig" but as a gay man he is also taking it to prevent him contracting the human immunodeficiency virus that can lead to Aids.

He takes Truvada, a pill introduced by the US drug company Gilead in 2004 as an antiretroviral treatment for people with HIV but since developed for pre-exposure prophylaxis - generally known as PrEP - for those at risk of infection.

Around the world, hundreds of thousands have been taking PrEP on a daily basis for several years, yet Dr Nutland, who bought his blue lozenge-shaped pills online, is only one of a handful of people in the UK taking the medicine.

The question for many - both activists and clinicians - is whether greater use of PrEP alongside more widespread diagnosis and early treatment could finally herald the end of HIV/Aids. The combination of the prophylactic for those who are HIV-negative with effective antiretroviral drugs, which suppress the virus in those who test positive, offers a chance to manage the disease out of existence, suggests Tony Fauci, director of the US National Institute of Allergy and Infectious Diseases.

NIAID, with other public agencies and companies, is developing long-acting forms of HIV prevention with formulations of antiretroviral drugs that need only be administered monthly or even annually. These include intravaginal rings, slow-release implants and long-acting injections. Viiv, GlaxoSmithKline's majority-owned subsidiary focusing on Aids, is carrying out clinical trials of a new antiretroviral called cabotegravir, given as monthly injections on its own or combined with other drugs.

But PrEP drugs cannot put an end to HIV/Aids on their own, Dr Fauci warns. They will have to be coupled with other approaches including a vaccine that prevents infection. "If deployed alongside our current armoury of HIV prevention tools," he says, the combination "could be the final nail in the coffin for HIV".

Funding debate

Truvada was first licensed for PrEP by the US Food and Drug Administration in July 2012, and subsequent studies in San Francisco and New York have seen a dramatic reduction in the transmission rates of HIV among gay men in those cities. Truvada, which is a combination of two HIV medications, tenofovir and emtricitabine, is 86 per cent effective at blocking the pathways that HIV uses to set up an infection, according to Aids.gov, a US government website.

In the US, a course of 30 PrEP tablets costs just over $US1,500, most of which insurance companies will cover - justifying the expense as cheaper than that of covering people living with HIV/Aids.

Infections in San Francisco dropped 17 per cent in 2015 compared with the year before, down from 309 to 255, after an 18 per cent annual fall in 2014. The city authorities ascribe the decline to its "Getting to Zero" campaign, a policy of promoting early diagnosis and rapid access to antiretroviral therapy for those who are HIV positive, ensuring infected people remain in care, and extending PrEP use to those who are negative but are deemed high risk. Approximately 20 per cent of the city's 30,000 gay and bisexual male population are thought to be using PrEP.

The US is not alone. France, Norway, Australia, Israel, Canada, Kenya and South Africa have all approved PrEP use.

In the UK, the treatment has become the latest battleground in the funding of drugs to prevent the spread of HIV/Aids. In November, the Court of Appeal ruled against the National Health Service, which had contested a High Court judgment that would force it to offer PrEP on request. The NHS estimates this would cost £10m-£20m a year. A daily course of Truvada costs about £400 a month.

The National Aids Trust, which took the NHS to court, suggested that the overall cost of caring for someone with HIV or Aids was much higher. A 30-year-old man infected today is likely to cost the health service in the UK almost £400,000 over his lifetime. Given that there are approximately 104,000 people in the UK living with HIV, the total bill, without taking into account new infections, could cost the NHS £41.6bn.

Just before Christmas, the NHS announced a £10m, three-year clinical trial, for 10,000 HIV-negative participants to gauge the efficacy of PrEP.

The NHS's sexual health clinic in London's Dean Street is the only NHS clinic offering PrEP at cost price in the UK. Over the past 12 months, it has seen a fall of 40 per cent, from 626 in 2015 to 373 last year, in the number of transmissions it has recorded.

"We were determined that 2016 would be the year that London learnt from San Francisco's success," says Dr Alan McOwan, the centre's lead clinician. "This drop in new HIV diagnoses could be really significant. The impact could spread across the UK thanks to better HIV awareness, frequent testing, early treatment and use of prevention methods such as PEP [post-exposure prophylaxis] and PrEP in key populations."

Slowing transmission

For the pharmaceuticals industry, HIV/Aids is a growing market. The Joint UN Programme on HIV/Aids estimates that the number of people worldwide on antiretroviral therapies rose by 1m during the first half of 2016, reaching 18.2m. Gilead, the leading provider, reported total product sales of $32.2bn for 2015, up 31 per cent on the year before. Truvada sales in the US alone grew 15 per cent to $2.1bn in 2015, with the global figure hitting $3.5bn.

According to Datamonitor Healthcare, which tracks the sector, approximately one-third of Truvada's use in the US was attributed to PrEP, rather than antiretroviral treatment. As yet, Truvada is the only PrEP drug recognised by world health organisations.

While transmission rates around the world are falling they remain stubbornly high. Globally, UNAids reports that there were 36.7m living with HIV at the end of 2015, double the numbers on antiretroviral therapy. An estimated 2.1m people became HIV positive in 2015, down from 3.2m in 2000.

Sub-Saharan Africa remains worst affected, with UNAids estimating in 2014 that 4.4 per cent of the adult population live with HIV. Nearly 70 per cent of those infected with HIV worldwide live in the region and they are gaining access to antiretroviral therapy, but not as comprehensively as in developed countries. UNAids warned in 2014 that 54 per cent - some 19m people globally - remain unaware of their status.

With so many infected, questions remain of how struggling health services can afford the burden of providing PrEP on demand. Yet for Ford Hickson, assistant professor in health promotion at Sigma Research, part of the London School of Hygiene and Tropical Medicine, this misses the point.

"In the public domain, in newspapers and the media, the way it [PrEP] is debated is not about reducing misery and keeping people healthy; the debate is about saving money and whether we should spend money on these people," he says.

For Dr Hickson and many activists, the argument in the UK over PrEP provision has unfortunate moral overtones of the early 1980s, when health services around the world were slow to respond to the growing issue of HIV/Aids. But while activists and clinicians argue about the costs and benefits of PrEP, many people around the world are still becoming infected with the virus.

Testing and practice

Philip Christopher Baldwin discovered he was HIV positive in 2010. Aged 24, he had just started working in the City of London as a lawyer.

"I thought at the time it might completely destroy my life," Mr Baldwin says. "I knew very little about what it was to be HIV positive, and then went through a process of acceptance coming to terms with my HIV and that took several years."

While the diagnosis may no longer mean a death sentence, for many the stigma remains. "There is still a massive amount of work to be done around HIV awareness," he adds.

The dilemma facing both pharmaceutical companies and health organisations is how to ensure people get tested and to determine the best method of prevention.

Some believe the combination of PrEP and the promotion of early testing and diagnosis could be the answer to reducing HIV transmission.

This is at the heart of the debate surrounding PrEP: is it a promiscuity pill? Would it encourage those in the most at-risk group - gay men - to ditch condoms altogether?

Certainly, some members of the gay community think it could. Michael Weinstein, head of the Los Angeles-based Aids Healthcare Foundation, which last year issued 14,000 HIV tests and gave away 38.5m condoms, initially declared PrEP to be a "party drug" and a "public health disaster in the making". Although the foundation has since made clear that it is not against PrEP, Mr Weinstein suggests that those most at risk will not adhere to taking a pill a day.

Sigma's Dr Hickson says: "What we need to see are changes in testing policy and, consequently, practices. It's not about the psychology of the men involved, but about the behaviour of the service - whether you get offered PrEP or not."

Discussing the question of promiscuity among gay men is fraught with difficulties. Accusations of homophobia are easy to level, and questions about whether gay men are more sexually active than their straight counterparts have long proved controversial. Add a prevalence of recreational drugs among certain groups and the question of safe sex becomes even more pressing.

"Some people have portrayed PrEP use as irresponsible," says Dr Nutland. "But I think it's totally the opposite. It's an immensely responsible thing to do, particularly if you are one of those guys who doesn't consistently use condoms."

For Mr Baldwin, who is healthy - he sees his specialist every three to six months - and successful, there is no reason for others to experience what he went through, he says.

"We need to see PrEP available as soon as possible," Mr Baldwin says. "Every day, 17 people are diagnosed with HIV [in the UK] and PrEP is an important tool in the kit for confronting the disease."