Peptides for weight loss: Are they safe and how do they work?

Weight-loss injectables have surged in popularity in recent years, with social media buzzing about medications like Ozempic and their potential to help people shed unwanted pounds.

Originally developed to treat type 2 diabetes, Ozempic contains semaglutide, a medication that has also been found to aid in weight management.

Semaglutide belongs to a broader class of compounds known as peptides, which are increasingly being explored for their role in obesity treatment and metabolic health.

Here's everything you need to know about weight loss peptides: How they work, potential benefits, possible risks, and whether they're a safe and effective option.

What exactly are peptides?

Peptides are short chains of amino acids that make up proteins in the body, according to Healthline.

There are different types of peptides that do a variety of things including "help orchestrate hormone activity, immune defense, tissue repair and growth, control of inflammation, and digestion," Harvard Health Publishing said.

And then, of course, there are peptides used to treat diabetes and obesity.

Weight-loss peptides, like the aforementioned semaglutide (a glucagon-like peptide-1 receptor agonist a.k.a. GLP-1) and tirzepatide (a dual glucagon-like peptide-1 and glucose-dependent insulinotropic polypeptide receptor agonist), are typically prescribed to people who are overweight or living with obesity.

In an interview with GMA News Online, Dr. Macgerald Cueto, a physician at Marie France, said peptides help people lose weight by:

• suppressing appetite
• slowing gastric emptying
• enhancing energy use

Cueto said these injectables have become increasingly popular in recent years because of their “unprecedented effectiveness” and because they address the biological causes of obesity.

“Obesity is more than a matter of willpower or discipline,” Cueto said. “It’s complicated.”

“It's often not addressed only through lifestyle modification alone but needs a more comprehensive and systems approach,” he added.

Dr. Kaycee Reyes, a physician and founder of Luminisce, said people living with obesity often have a weak signal of when to stop eating.

“Every time you eat, our body releases hormones that travel up to your brain and basically say, ‘Okay, we’re good, you can stop now,’” Reyes explained.

“In a lot of people living with obesity, that signal is weak,” she added. “It’s not a willpower problem. The message just isn’t getting through clearly.”

Who should use weight loss peptides?

Although these medications are approved to treat specific medical conditions, some people use them solely for weight loss despite not having obesity, a practice Cueto cautioned against.

“These medications are not appropriate for everyone who wants to lose weight,” he said, explaining "they are intended for specific patient populations with particular medical indications.”

These medical conditions include:

• People who are overweight (BMI of 27 or higher) with prediabetes
• People with cardio-kidney-metabolic syndrome
• People with type 2 diabetes
• People with obesity (BMI of 30 or higher)

On the other hand, these medications should not be used by people with medullary thyroid carcinoma or Multiple Endocrine Neoplasia Type II (MEN II).

However, Reyes emphasized that common thyroid disorders are not contraindications.

“Hyperthyroidism, goiter, papillary or follicular thyroid cancer — none of these disqualify you,” she said. “It’s specifically medullary thyroid carcinoma and MEN II.”

For people who do not meet the recommended criteria, both physicians warned that the long-term effects remain unknown.

“We have no idea what these drugs do to a normal-weight body over years, because that study has never been done,” Reyes said. “I’m not going to make my patient the experiment.”

Are they effective? What are the side effects?

Bariatric surgery aside, Reyes described weight-loss peptides as "the most effective weight-loss medicine we've ever had."

Meanwhile, Cueto said clinical data show these medications can help patients lose around 15% to 20% of their body weight after 52 weeks of continuous treatment at higher doses, noting that everybody is unique and some respond faster than others.

The most common side effects are gastrointestinal, including nausea, vomiting, constipation, diarrhea, and bloating. Some patients also experience dehydration and fatigue.

Cueto said that there are also rare but serious side effects requiring medical attention, including acute pancreatitis, gallstone formation, hypoglycemia, lean muscle loss, and severe dehydration.

“These are often seen during rapid weight loss,” he said. “It’s always prudent to be under the direct guidance of a medical physician.

Do you regain the weight if you stop the medication?

According to Reyes, regaining weight is expected after discontinuing treatment.

“I want to be very direct about that, because this is where people feel betrayed if nobody warned them,” she said.

She pointed to clinical studies showing that the benefits of these medications are difficult to maintain after treatment ends.

In one study on semaglutide, participants lost around 17% of their body weight, but a year after discontinuing the medication, they had regained about two-thirds of the weight they had lost.

“Nobody is shocked that blood pressure goes up when you stop blood pressure medicine,” Reyes cited as an example. “We understand that the drug was controlling the condition, not curing it.”

She said obesity should be viewed the same way as a chronic disease.

“Stop the medicine, and the condition returns,” she said. “That’s not a failure of the drug. That’s the definition of a chronic disease.”

Peptides are not a replacement for diet and exercise

Cueto said that one of the biggest misconceptions circulating on social media is that these medications are a shortcut that can replace healthy eating and exercise.

“Medications work best as part of comprehensive lifestyle management,” he said. “They are tools that make behavior change easier, not a replacement for healthy habits.”

Reyes also noted that some muscle loss is expected while taking weight-loss peptides, making strength training especially important.

“Muscle is your metabolism, your strength, your independence when you’re old,” she said.

“If you take this drug, eat like a bird, and never lift anything, you’ll be thinner and weaker,” she added.

She recommends eating protein with every meal and doing resistance training two to three times a week.

“I’d rather a patient lose 12% and keep their muscle than lose 20% and be frail,” she said.

Risks of buying over-the-counter peptides

Cueto stressed that weight-loss peptides should only be used under a physician's supervision.

“Without medical supervision, one often misses critical monitoring for dose-related adverse effects,” he said.

“Medical supervision provides dose adjustment, monitoring for dangerous complications, and access to quality-controlled medications,” he added.

In the Philippines, Reyes noted that weight-loss peptides are prescription-only medications regulated by the Food and Drug Administration.

“So, if a seller is offering you a branded product with no prescription, that transaction is already outside the law,” she said.

Reyes also warned consumers about products labeled "research grade."

“Research grade is not an actual grade,” she said. “It’s a phrase that exists so a seller can avoid the word medicine.”

Both Reyes and Cueto emphasized that weight-loss peptides should only be taken under medical supervision.

In 2025, the World Health Organization recommended a range of blockbuster weight-loss drugs to treat diabetes and obesity globally.

The WHO added semaglutide—the active ingredient in Novo Nordisk's Ozempic and Wegovy—and tirzepatide, the active ingredient in Eli Lilly's Mounjaro, to its list of essential medicines for adults.

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