Phentermine & Qsymia
While there is no publicly available information to fully answer your question, we've used the available data to pull together key findings: In general, it is the practice of physicians to only prescribe Phentermine for a period of three months, in accordance with both FDA guidelines and a general belief that the drug loses its effectiveness after that point (a belief that may be contradicted by a very recent study). While intermittent use (i.e., discontinuing use for a time before getting another prescription) is common, it is not known exactly how common. Qsymia, on the other hand, is intended for life-long use. Consequently, there are no guidelines against using it long-term and no follow-up studies on how long the average patient takes it, though indications are that the majority do not remain on it indefinitely.
Below you'll find an outline of our research methodology to better understand why the information you've requested is publicly unavailable, as well as a deep dive into our findings.
We began by reviewing our colleague's prior work, both to see what they had already discovered and to ensure that we wouldn't simply reduplicate their research strategy. While it is nominally Wonder's practice to restrict our research to sources published within the last two years, given the lack of information found in our original attempt, we expanded our criteria to include older sources, though of course giving preference to the latest available data. Moreover, while our focus remained on the US, we did not overlook research published abroad which might assist in answering the question. Finally, while of course giving preference to academic works, we also searched less formal articles, white papers, and even blog posts by physicians in order to get a better picture of the "on the ground" use of Phentermine and Qsymia.
PHENTERMINE METHODOLOGY AND FINDINGS
This led us to an undated blog post in which Dr. Westin Childs discusses proper vs. improper use of Phentermine as a weight loss supplement. Specifically, he expresses concern that so many patients rely on the drug's ability to suppress the appetite without making changes in their lifestyles, such as developing an exercise regimen. The reason that this is a concern is that most doctors "will NOT prescribe Phentermine longer than a 3-month time period because it is well-known that the benefits of Phentermine start to decline after that time period." He offers as an example that if Phentermine helps a patient to lose 5 lbs. in the first month, it will only help them lose 2-3 in the second month and perhaps 1 in the third month--and after that, none at all. This limit of three months was noted by US News and World Report in a 2015 article, and so seems to be fairly common knowledge.
The study includes several other salient details about Phentermine use: First, that the FDA-approved use of the medication is limited to less than 112 days. Second, that in "clinical practice, patients often take Phentermine intermittently over time." That is, they go through an initial regimen, cease using the drug for a time, then get a new prescription. Thus, the study group was divided into "on-label" short term users, and "off-label" medium term (up to 6 months) and long-term (up to 1 year) users, with each category further divided into intermittent and continuous use.
What this study confirmed is that the approved practice is to only prescribe Phentermine for three months, but that it is common for patients to go off the medication for a time and then get a new prescription. Because the very nature of the study meant that it went outside the FDA-approved use of the drug, we are reluctant to draw conclusions about how common intermittent use for longer than 112 days is from it. Having said that, here is the study's breakdown of the participants:
Short-term referent - 6,764 participants
Short-term intermittent - 2,938 participants
Medium-term continuous - 7,703 participants
Medium-term intermittent - 2,423 participants
Long-term continuous - 144 participants
Note that these are overlapping categories due to the nature of the study, e.g., the long-term continuous patients are also counted among the medium-term patients because they were evaluated at both stages. However, it is not entirely clear from the study how the overlap should be accounted for, so we have not attempted to break these categories into percentages.
Apart from the above, there is no further data in the public domain which indicates how common it is for doctors to re-prescribe Phentermine "intermittently" beyond the initial three-month period permitted by the FDA. Based on the available information, we can only say that three months is the most common prescription period, with the prescription requiring renewal every 30 days.
QSYMIA METHODOLOGY AND FINDINGS
Wheras Phentermine's use is restricted to short-term prescriptions, the official FDA data sheet on Qsymia recommends discontinuing use after 12 weeks if certain weight-loss thresholds are not met, but it does not otherwise limit how long the drug may be administered. In fact, the original announcement by the FDA that it had approved the drug (published in 2012) indicates that Qsymia was, at the time, one of only three drugs at the time approved for "long-term weight management." In fact, Qsymia is intended to "be taken for the rest of a person's life."
We first studied the available peer-reviewed literature on Qsymia in the hopes of finding a study on its long-term effects which would include information on how long the study subjects had taken the medication. If such a study exists, the breakdown of the study participants was not evident in any of the available articles or article abstracts, though we conceed the possibility that the data could be buried in the text of a paper that has not been fully published to the public domain.
As in our general methodology, we also looked to non-academic medical sources, including but not limited to Dr. Childs' site. At least one patient tells Dr. Childs that they have been on Qsymia for a year, though they have reduced their dosage to 2-3 times per week. Another news article refers to a patient who had (in 2017) been on Qsymia for two years. But again, anecdote is not data, and while Dr. Childs' does comment on the effectiveness of Qsymia and once again cites peer-reviewed paper, it's interesting that the only source he can cite for Qsymia's effectiveness for up to 56 weeks is the drug's own homepage. However, unlike in the case of Phentermine, he makes no comment on how long he or his colleagues commonly prescribe it for. A broad-based search of healthcare sites and news media sources failed to locate any additional sources with more information from which we could even triangulate an answer.
Finally, as a last-ditch effort to provide the requested information, we studied the annual and quarterly reports of Vivus, the pharmaceutical company which produces Qsymia. While they refer to the drug repeatedly in their report, including revealing that the number of prescriptions is in decline (from 566,000 in 2015 to 395,000 in 2017), there is no indication that the company itself is aware of how long the average person uses Qsymia. At most, we can infer that users are not staying on the drug for the "life-long" term approved by the FDA.
While the FDA only approves of Phentermine for short term (under 112 days) use and physicians say they usually prescribe it for only three months at a time, it is known that many patients use the drug intermittently. However, how many patients do so is unknown, with the only study on the subject deliberately selecting for long-term use. In the case of Qsymia, it has been approved for long-term, even life-long, use by the FDA, but falling prescription counts indicate that most patients do not remain on it indefinitely. While there are anecdotal accounts of patients remaining on it for two years or more, there is insufficient public data to determine what the average regimen term might be.