Research ethicists, commenting in the latest edition of the British Medical Journal, have expressed concern at what they call the 'cynical use' of these studies for marketing purposes.
One of the problems cited by Dr Gordon Taylor from the University of Bath and co-author Professor Paul Wainwright from Kingston University was that patients volunteering for the trials tended to be a self-selecting group with those unlikely to benefit from the new group dropping out following clinical trials.
In addition, researchers investigating open label extension studies (OLES) estimated that less than four per cent of these studies were published in scientific journals and that many of the trials they looked at were of dubious scientific integrity.
"Open label extension studies are used to gather information about the safety and tolerability of the new drug in the long term and through day to day use, but less than four per cent of these trials are ever published," said Dr Taylor who is a member of the Association of Research Ethics Committees.
"Dressing up marketing exercises in the regalia of research lends them a spurious authority, allowing participants and clinicians to believe they are contributing something worthwhile to science, rather than simply boosting the market share for the relevant pharmaceutical company," he added
As part of the R&D process, pharmaceutical companies often carry out OLES, which enable them to collect data after clinical trials have been carried out but before the drug receives final approval.
The problem is that many of the patients who volunteer to take part in these studies have previously been involved in clinical trials of the product and are often recruited on the basis that they are helping scientific research.
They also question the ethics of recruiting patients who were involved in 'blind' trials of a new drug - where they don't know if they are receiving the drug or a placebo - as they may decide to take part in the study because the new drug, or placebo, made them feel better without ever really knowing which they received.
"We are not suggesting that all studies are necessarily unscientific or unethical, but it is our experience that a considerable number of studies raise significant concerns," Taylor said.
"These studies appear to be particularly prone to the pressures of marketing of good research methodology and research ethics," he added.
As many of these studies are carried out whilst clinical trials are still ongoing, it is not always clear that the new drug is actually an improvement on existing treatments that are available.
Some patients may be involved in OLES for as much as two years - or even longer in some cases - having only been involved in clinical trials for a few weeks.
They argue that, where OLES can be useful is in giving patients access to beneficial drugs when no suitable alternative is available. However, they say that this 'compassionate prescribing' of new drugs should not be facilitated through research studies.
"Some of the problems with these studies could be resolved by letting patients know whether they have been receiving the new drug or a placebo in phase three of the clinical trial," said Taylor.
This will make sure that they can make a decision on whether to continue in the open label extension study based on their experiences from the trial.
Taylor concluded that if these studies were to go ahead under the name of research, then there should be a much more rigorous application of scientific principles in their design and implementation that would allow for variations in samples and other areas.