Businessmen in pharma may or may not be ethical, but the design of clinical trials and their conduct are overseen by both institutional review boards (IRBs) and FDA scientific review divisions, which are typically far down the chain of any agency bureaucratic interference and are firewalled from contact from the companies or academics conducting the trials. Every clinical trial protocol, or revision to that protocol, has to be approved by an IRB and, for an drug intended for eventual marketing, is reviewed by the most relevant FDA scientific division. Any study done in the US is also beholden to be conducted, by law, according Good Clinical Practice (GCP) standards, and outside the US, its equivalent is the International Council for Harmonisation of Technical Requirements for Pharmaceuticals for Human Use (ICH), which has similar standards that most all 1st world countries adhere to by law. If a study is not conducted according to these standards, the data cannot be used to support an approval of a drug product in these countries and is therefore useless. Independent Data Monitoring Committees are also used to examine unblinded data as it comes in during the trial. These committees help determine whether to terminate the study early for either safety or ethical reasons (such as either negative or very positive results). Serious, unexpected adverse events reported by investigators participating in these trials are also required to submitted by companies within 7 to 15 days, depending on severity, to the FDA.
Bottom line, unethically designed clinical trials either are prevented before initiation by IRBs or, for first in human trials, the FDA scientific review divisions, which can also stop them at any time, by using its clinical hold powers. All trials in the US are also required to be registered and posted at clinicaltrials.gov for the public to search and obtain information about. The oversight and legal requirements of ethical and risk/benefit considerations of trial design doesn't stop all bad behavior, as there certainly are cases where companies have been found to falsify, cherry pick, or suppress data. These issues are usually discovered during inspections of clinical site records, which are required during the drug approval process, and usually such anomalies eventually reveal themselves in the data and disqualify it. There is also regular communication between the FDA and its European equivalent, the EMA, which share information about such things. Clinical trials are expensive to conduct. Companies, for the most part, can't easily operate trials in a rouge fashion, as data obtained in such a way will be unusable to obtain marketing approval anyway, and open them to major financial and litigative risk, and because of legal restrictions under the US Food, Drug, and Cosmetic Act. Doesn't stop all intentional bad actors from doing intentionally deceptive things, but it severely curtails such things in the US and the rest of the developed world.