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A vote by British citizens to leave the European Union could have important implications for the way drugs and medicines are tested and marketed.

The referendum is being watched closely by many leading players in the pharmaceutical industry who argue that a Brexit vote would be damaging for research and patient access.

But some campaigners have argued that drugs could be made available more quickly if the UK votes to leave.

Each EU member state has its own medical regulatory body, in the UK's case the Medicines & Healthcare Products Regulatory Agency (MHRA).

Companies wanting to sell new drugs or devices in this country have to get them licensed by the MHRA. But an alternative route for major pharmaceutical companies is to get approval from the European Medicines Agency (EMA), which is a decentralised agency of the EU.

Through this centralised procedure, companies can get the go-ahead for medicines to be marketed across all member states. Typically, manufacturers of drugs to treat cancer and rare diseases use this central approval route.

The EMA is run by regulators from member states who staff its committees so there is input from across the EU for each approval. That means there does not need to be a separate national process and, once granted by the EMA, the centralised authorisation is valid both in member states and countries in the European Economic Area (EU plus Iceland, Liechtenstein and Norway). The EMA is based in London.

So what happens if there is a vote for the UK to leave? It seems certain that the EMA would move its headquarters out of London to an EU country. Some in the pharma world argue that this will in itself reduce the importance of this country in the eyes of the global drug companies.

If the UK decides to negotiate to stay in the EEA there would not, in practice, be much difference to regulation. But if the UK stays out of the EEA, drug companies would need to go through a separate process with British regulators for new products as the centralised European route would not be applicable to the UK.

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Most leading players in the world of big pharma have come out in favour of Britain remaining in the EU, arguing that being outside the EMA process will deter manufacturers from selling some new drugs in this country. The other side of the argument is that Switzerland is not a member of the EEA and has a well-established pharma industry with big names such as Hoffman-La Roche and Novartis.

Drug trials are currently carried out on a national level, with companies needing to approach each regulator and ethics committee separately. There is a move to harmonise this procedure across the EU over the next few years, allowing a single entry point for companies that wish to carry out trials of a new drug on patients in different countries.

The Leave campaign argues that the existing EU Clinical Trials Directive has damaged medical research and innovation in the UK. The Commons Science and Technology Committee said in a recent report: "Weaknesses in the 2001 Clinical Trials Directive significantly increased the administrative burden and cost of running academic clinical trials and saw a reduction in trials taking place in Europe." But it went on to add that the new regulations due to take effect in 2018 appeared to be an improvement.

The pharma industry argues that the UK is involved in about 40% of all adult rare diseases trials in the EU at present, but this would be undermined by a change of status. Being outside the EU, the industry claims, would mean the UK was not part of the harmonised procedure and so might lose out on some trials that might otherwise benefit patients.

Officials at the National Eczema Society have said they have been informed by two US companies that trials of new treatments would not take place in the UK in the event of Brexit.

But the EMA has been criticised for a bureaucratic and slow-moving process, which has not helped patients desperate for life-preserving drugs. Recent media reports suggested that European approval of a breast cancer drug, palbociclib, which helps the slow the development of tumours, had been held up at a time when US regulators had fast-tracked its adoption.

The industry points out, however, that the approval documentation was submitted to EU regulators 10 months after the American Federal Drugs Administration.

Few would disagree that the result of the referendum vote is important for drug companies and their products. Whatever the outcome, though, it could take years rather than months for the implications to become clear.

Read more: The facts behind claims in the EU debate