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Cosmos Study

Patients Who Lose Less Than 20 Pounds on GLP-1 Treatment Lose More Weight After Switching to Phentermine

May 16, 2024
Dual-Team Study
Team A:Kersten Bartelt, RNBrendan Joyce
Team B:Jeff Trinkl, MDJoe Deckert, PhD

Key Findings

  • Patients who lost less than 20 pounds on GLP-1 medications and were subsequently prescribed phentermine experienced greater weight loss while on phentermine in the year following GLP-1 treatment than patients who lost at least 20 pounds on the GLP-1 medication. 
  • Patients prescribed phentermine after discontinuing their GLP-1 medication were 29% less likely to regain all the weight they lost on the GLP-1 medication one year after the end of their GLP-1 treatment compared to those who were not prescribed phentermine (19.3% vs. 27.3%). 

Phentermine, one of the earliest FDA-approved weight loss medications, functions by suppressing appetite.1 To determine whether patients who transitioned to phentermine after discontinuing a glucagon-like peptide-1 receptor agonist (GLP-1) medication maintained or experienced additional weight loss, we studied 7,773 patients who were prescribed phentermine following discontinuation of a GLP-1 medication. We categorized patients based on the amount of weight they lost while on GLP-1 treatment.  

Patients who gained weight or maintained their weight while on a GLP-1 medication were most likely to experience weight loss after switching to phentermine with a median weight loss of around 3% in the year after discontinuing the GLP-1 medication. Patients who switched to phentermine after losing at least 20 pounds while on the GLP-1 medication were most likely to regain weight one year after discontinuing their GLP-1 medication, with more than half regaining some or all their weight.  

Figure 1
Median Weight Change on Phentermine by Amount of Weight Lost on GLP-1 Treatment
Median Weight Change on Phentermine by Amount of Weight Lost on GLP-1 Treatment
Figure 1. The median percentage of weight change after switching from a GLP-1 medication to phentermine by the amount of weight lost while on the GLP-1 medication.

We then evaluated whether patients prescribed phentermine after discontinuation of a GLP-1 medication were more likely to keep their weight off one year later compared to those who did not start phentermine after discontinuing their GLP-1. We found that 27.3% of patients who did not use phentermine after their GLP-1 medication regained all the weight they lost while on the GLP-1 compared to 19.3% of patients who started phentermine after discontinuing their GLP-1. 

Figure 2
Total Weight Regain Rate by Phentermine Use
Total Weight Regain Rate by Phentermine Use
Figure 2. The rate of total weight regain by phentermine use after GLP-1 discontinuation. 

These data come from Cosmos, a dataset created in collaboration with a community of Epic health systems representing more than 246 million patient records from 1,400 hospitals and more than 32,500 clinics from all 50 states and Lebanon. This study was completed by two teams that worked independently, each composed of a clinician and research scientists. The two teams came to similar conclusions. Graphics by Mark LeBay. 

References

  1. Davidson K, MScFN, RD, et al. Phentermine for weight loss: Benefits, side effects, and dosage. Healthline. Published January 24, 2019. https://www.healthline.com/nutrition/phentermine-weight-loss. Accessed April 16, 2024.