Last year, the European Medicines Agency banned about 700 medicines across Europe after an investigation revealed data tampering in some trials of generic drugs in India. The questionable trials were conducted by “contract research organizations” or CROs in India on behalf of major international durgmakers located in other countries around the world.

International medical experts said that volunteers undergoing back-to-back clinical trials endangers the health of patients participating. It can also compromise clinical data gathered through these trials, on the basis of which drugmakers seek approval to sell generic medicines around the world.

“The time gap between participation in two different trials should be 90 days minimum,” said Stephanie Croft, a lead inspector at the WHO. “When [data] is incomplete or incorrect it could pose a serious risk to patients.”

Yet, some of the “volunteers” in India who participate in drug studies for international drugmakers who use Indian CROs are “addicted” to the money and find ways to violate the rules and do not wait the required 90 days between participating in clinical drug trials.

Half of more than a dozen volunteers interviewed by Reuters across four cities – Chennai, Hyderabad, Bengaluru, and New Delhi – said they waited much less than 90 days between trials. In the past three-to-four years, they said they spent several months at a time in different cities so that they could participate in as many studies as possible.

One volunteer, Vasudeva Prakash, a former mechanic, said he was never asked by CROs, and their ‘agents’ who approached him for studies, about whether he had recently taken part in another trial.

“Everybody does it. Once you start getting the money, it’s very hard to quit. It’s like an addiction,” said Prakash.

He said after the first study, he began to regularly receive messages on his phone and Facebook, often from agents working on behalf of CROs, informing him about ongoing clinical trials where volunteers were required. Such messages included three key things: the city where the trial was being conducted, the total pay offered, and the “blood loss”, or the amount of blood the volunteer will need to provide.

Prakash provided Reuters with documentation proving he underwent trials with short gaps at Apotex Research Pvt Ltd, owned by Canadian drugmaker Apotex Inc; Lotus Labs, owned by U.S. generics giant Actavis; Ethics Bio Lab, owned since last year by U.S. drugmaker Par Pharmaceutical Inc; and India’s Semler Research Center Pvt Ltd, among others.

In addition to the questionable status of volunteers, several CROs have been accused of outright data tampering. For example, CRO, Quest Life Sciences, was found last year to have manipulated clinical data on certain trials, according to inspection reports from the WHO and the UK’s medicines authority.

Several large international drugmakers, including Teva Pharmaceutical Industries Ltd and Mylan NV, rely on CROs in India to carry out tests on cheaper versions of branded drugs. The aim of these so-called “bioequivalence” studies is to gauge whether non-branded drugs are equally safe and effective. The faster the trials are undertaken, the faster the drugs can come to market.

In the wake of trial data manipulation scandals at CROs in the past three years, many large drugmakers including Swiss firm Novartis, have been shifting more critical trials back to the United States and Europe, according to consultants and industry executives.

International and local regulators have struggled to keep its oversight in line with the growth of an industry that expanded rapidly in the 2000s, as drugmakers shipped clinical trial work to India to save money. The market is estimated to have crossed $1 billion in 2016, according to consultants Frost and Sullivan.