Critical Assessment Found CB1 Blockers Were No Better Than Existing Weight Loss Drugs | Research

The takeaway

Despite promising mechanisms, CB1 receptor antagonists like rimonabant achieved weight loss comparable to existing anti-obesity medications while carrying potentially severe psychiatric side effects.

Read this if you're interested in why cannabinoid-based weight loss drugs haven't succeeded despite promising science.

CB1 antagonist weight loss did not exceed existing medications; psychiatric side effects limited use

What the researchers found

This critical review assessed how close CB1 cannabinoid receptor antagonists were to being ideal anti-obesity drugs.

The mechanisms were sound: CB1 antagonists reduced food intake centrally (brain) and may increase energy expenditure peripherally (thermogenesis in animal studies).

However, the clinical reality was disappointing. Despite these dual mechanisms, the weight loss achieved by rimonabant and taranabant (the two most advanced compounds) did not exceed that of already-approved anti-obesity medications.

More concerning, both compounds were associated with potentially severe psychiatric adverse effects including depression, anxiety, and suicidal ideation, which significantly limited their clinical utility.

The review noted several new CB1 antagonists in development and questioned whether they would differ meaningfully in efficacy and safety.

Why it matters

This review provided a reality check on the anti-obesity cannabinoid approach. While the mechanism was elegant, the clinical outcomes were underwhelming and the safety concerns were serious, leading to rimonabant's withdrawal and the shelving of other programs.

The numbers in context

Rimonabant and taranabant: weight loss did not exceed existing medications. Psychiatric adverse effects limited clinical use. Multiple new CB1 antagonists in development at the time.

How the study worked

Critical narrative review of CB1 receptor antagonist mechanisms of action, clinical trial efficacy data, and safety profiles, compared to existing approved anti-obesity medications.

What this study cannot tell us

Published during a period of active drug development, so the review could not assess compounds that were not yet in clinical trials. The comparison to existing anti-obesity drugs depended on the specific comparators chosen.

How to read the evidence

This is a critical review synthesizing clinical trial data, providing a moderate evidence-based assessment of the CB1 antagonist approach to obesity.

When this study was published

Published in 2009. Rimonabant was withdrawn in 2008 and taranabant development was stopped. Newer peripherally restricted approaches are still being investigated.

The bigger picture

This critical assessment foreshadowed rimonabant's market withdrawal in 2008 and the subsequent termination of taranabant's development. The field has since shifted toward peripherally restricted CB1 antagonists that may avoid brain-mediated psychiatric effects.

Questions still open

Can peripherally restricted CB1 antagonists achieve meaningful weight loss without psychiatric risks? Is the endocannabinoid system the right target for obesity, or are the psychiatric risks inherent to CB1 modulation?

Common questions

Why don't CB1 blockers work better for weight loss?

The body has multiple redundant appetite regulation systems. Blocking one (the endocannabinoid system) may not produce dramatic weight loss because other systems compensate. The brain also resists weight loss through various adaptive mechanisms.

Are there any cannabinoid-based weight loss drugs available?

No. Rimonabant was the only approved CB1 blocker and was withdrawn in 2008 due to psychiatric side effects. No replacement has been approved. The field is exploring peripherally restricted compounds that may avoid brain-related side effects.

Read the original research

A critical review of the cannabinoid receptor as a drug target for obesity management.

Obesity reviews : an official journal of the International Association for the Study of Obesity, 10(1), 58-67

View on PubMed ↗Publisher’s paper ↗

Citation

Akbas, F; Gasteyger, C; Sjödin, A; Astrup, A; Larsen, T M. (2009). A critical review of the cannabinoid receptor as a drug target for obesity management.. Obesity reviews : an official journal of the International Association for the Study of Obesity, 10(1), 58-67. https://doi.org/10.1111/j.1467-789X.2008.00520.x