Plenity (Gelesis100)

What it is and how it works

Plenity is not a drug. It is three capsules swallowed with water 20 to 30 minutes before lunch and dinner. Inside each capsule are hydrogel particles made from cellulose and citric acid that absorb water, take up space in the stomach and small intestine, and then break down and pass out of the body. The FDA cleared it in 2019 as a Class II device for adults with a BMI of 25-40, used together with diet and exercise, and cleared it for non-prescription sale in January 2024.

Each capsule holds about 0.75 g of absorbent hydrogel particles roughly the size of a grain of salt. The capsule disintegrates in the stomach and the particles hydrate up to 100 times their original weight; fully hydrated, one 2.25 g dose occupies about a quarter of the average stomach volume and mixes with food to create a larger, more elastic volume. The particles are not absorbed by the body and leave in normal bowel movements.

Because nothing crosses into the bloodstream, the FDA regulated it as a device rather than a drug, and the bar it had to clear was a device bar. That distinction runs through everything else on this page: the evidence, the way the benefit is described, and the fact that insurance generally does not treat it like a medication.

What the trial found

One 24-week trial carries the clearance. It randomised 436 adults aged 22 to 65 with a BMI of 27 to 40 at 33 sites in the US and Europe, and compared Plenity against identical capsules filled with raw cane sugar. Everyone in it was also prescribed reduced calories and exercise.

Put another way: in the published report, 59% of the Plenity group lost at least 5% of their body weight and 27% lost at least 10%, against 42% and 15% of the placebo group. The odds of reaching either threshold were roughly doubled. The authors also reported that participants who were prediabetic or had untreated type 2 diabetes at the start had around six times the odds of reaching 10% — a subgroup result, and one to hold lightly: the FDA review noted that the trial’s first secondary endpoint, which covered people with impaired blood glucose, did not reach statistical significance, so everything after it was treated as description rather than proof.

The trial also has to be read with its own caveats, most of which come from the FDA review rather than from critics. There were two co-primary endpoints. The responder endpoint was met — the 58.6% of the Plenity group reaching 5% weight loss comfortably beat the 35% target the trial had set itself. The other endpoint was not: Plenity had to beat the sugar capsules by 3 percentage points of body weight and managed 2.1. Nearly a quarter of the Plenity group and 29% of the placebo group left before the end, and the FDA wrote that the resulting missing data made the main analysis more dependent on the statistical model used. When the agency split the results by region, the difference between Plenity and placebo was smaller at the US sites than at the European ones, which led it to write that patients in the US may see less benefit than European patients did.

A small extension followed 39 people for a further 24 weeks, and only people who had already lost at least 3% of their body weight in the main trial could join it — so it describes how those who responded got on, not how everyone would. The 21 who had been on Plenity all along had reached 7.1% weight loss by the end of the main trial and ended at 7.9% after about 48 weeks in total. Beyond that there is no long-term data, and no trial has compared Plenity head to head against a weight loss medication.

Who the clearance covers

The FDA's indication is one sentence: Plenity is indicated to aid in weight management in overweight and obese adults with a BMI of 25 to 40, when used in conjunction with diet and exercise. Two details in that sentence do real work. "Aid in weight management" is deliberate — the FDA wrote that the level of effectiveness shown was not appropriate for an indication of weight loss, but was enough for an aid to weight management. And "in conjunction with diet and exercise" reflects the trial, where every participant was on a nutrition and exercise programme too.

The 2024 clearance carries the same indication in slightly updated wording — "adults with excess weight or obesity" rather than "overweight and obese adults" — and the same BMI range of 25 to 40.

Note also that the clearance starts at a BMI of 25, lower than the threshold for most weight loss medications, while the trial itself enrolled from a BMI of 27 upwards, and anyone in it with a BMI under 30 had to have type 2 diabetes, high cholesterol or high blood pressure as well. So the bottom of the cleared BMI range is wider than the group the evidence was gathered in.

Side effects, with rates

The side effects are digestive and mostly mild, and the overall rates were close between the two groups — 71.3% of the Plenity group and 70.6% of the placebo group reported at least one adverse event. What differed was the gut-specific ones:

No one in the Plenity group had a serious adverse event during the trial and there were no deaths in either group. Two cases were severe — one of abdominal distension and one of nausea — and both resolved when the person stopped using the device. Withdrawals because of side effects were 3.6% on Plenity and 3.3% on placebo. A sub-study at four sites measured vitamin levels and found no significant differences from baseline or from the placebo group, and a separate crossover study in small groups of eight male volunteers found no change in the rate at which the stomach emptied.

Cost and availability

On price there is no manufacturer list price to quote. The only figure in a dated, citable publication is more than four years old — American Family Physician reported $98 for a one-month supply in March 2022, which is a March 2022 figure and not a current price, and it is deliberately left out of the comparison tables on this site for that reason. The prices quoted directly to consumers in September 2026 differ from that figure and from each other, with discounted first-month plans sitting well below the stated full price, so the number you are shown depends on where you look and when. Because Plenity is a device rather than a drug, it has generally not been covered by insurance.

Retail listings are marketing material, not a regulatory record, and at least one of them carried a weight-loss figure larger than the trial reported. The numbers on this page come from the FDA review and the published trial instead. A pharmacist can tell you how the timing rule interacts with anything else you take, which is the part of this that is easy to get wrong.

How it sits in the wider evidence

Plenity occupies an unusual space. It is sold over the counter, but unlike a supplement it went through a randomised, placebo-controlled trial and an FDA review, and both are public documents anyone can read. It is also not a medication, and the size of the effect it demonstrated sits far below what the GLP-1 drugs reported in their own trials. Those are different trials in different populations, and the comparison is not a fair one in either direction, but the order of magnitude is worth knowing.

The FDA's own conclusion was that one of the two co-primary endpoints was met, "demonstrating a modest clinical benefit in weight loss", that there is "moderate uncertainty in the benefit", and that because the adverse-event profile was very low the benefit-risk balance came out favourable. The side effects the trial recorded were digestive: bloating, diarrhoea, changed bowel habits, wind.

Whether it belongs in anyone's plan is a conversation for a clinician or pharmacist, and being able to buy something without a prescription does not make that conversation less useful — particularly around the timing rule and other medicines.

Common questions

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Last reviewed: September 2026