How long does it take for Mounjaro to work?

While the primary goal of diabetes medications is to regulate blood sugar levels, some have the added benefit of helping with weight loss. One such medication is Mounjaro (tirzepatide), a GLP-1 receptor agonist manufactured by the pharmaceutical company Eli Lilly. It stimulates insulin production, decreases appetite, and slows digestion. In addition to improved glucose control, it commonly helps patients reduce their food intake. That’s why even though the medication is only approved by the Food and Drug Administration (FDA) for Type 2 diabetes, it’s also used off-label as a weight-loss medication. 

The once-weekly injectable prescription drug Mounjaro works quickly—you may notice decreases in appetite almost immediately and improvements in blood sugar and weight within a few days to several weeks—though the rate of weight loss slows after this initial period. Keep reading to learn what to expect if you’re prescribed this medicine.

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How fast does Mounjaro work?

You’ll notice decreased hunger and food cravings almost immediately after you take Mounjaro, says Alexander Nathanson, MD, an obesity medicine physician at NYC Health + Hospitals/South Brooklyn Health in New York. For example, you may notice that you’re unable to finish the same portion sizes you used to. However, it may take several days or weeks after starting Mounjaro to notice changes in your blood sugar control and weight. 

Blood sugar

Patients using Mounjaro for diabetes should start to notice some improvement in their blood sugar control within days of starting a Mounjaro prescription. “Mounjaro allows the pancreas to produce insulin in a more efficient or ‘dose-dependent’ manner in response to food—a process that’s often impaired in those with Type 2 diabetes,“ Dr. Nathanson says. 

Sustained changes in blood glucose levels are likely within one to two weeks of starting Mounjaro, says Kevin Codorniz, MD, the division chief of endocrinology at Loma Linda University Health in California. However, some patients may have a more delayed response to the medication or require a higher dose of Mounjaro in order to see significant effects. 

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Weight loss

“After the first dose of tirzepatide, patients may already experience a decrease in appetite,” Dr. Codorniz says. And while every individual is different, many patients will start to see some weight loss within the first four weeks of taking tirzepatide, he says. 

Mounjaro mimics the action of glucagon-like peptide-1 (GLP-1) and gastric inhibitory polypeptide (GIP; also called glucose-dependent insulinotropic polypeptide), two naturally occurring hormones produced by the gut. Within minutes of eating, GLP-1 and GIP are naturally secreted in the intestines, stimulating the release of insulin and signaling satiety (reducing “food noise”) in the brain. The hormones also delay stomach emptying by slowing down gastrointestinal transit time, Dr. Nathanson says.

Patients can lose a noticeable amount of weight within the first few weeks of using the medication, though the rate of weight loss does often slow down going forward, explains Dr. Nathanson. 

A 2022 SURMOUNT-1 trial published in the New England Journal of Medicine examined the effects of tirzepatide on patients with obesity. Along with lifestyle modifications, including regular exercise and a healthy diet, study participants received weekly single-dose subcutaneous injections of 5 mg, 10 mg, 15 mg, or a placebo (placebo means inactive medicine). After 72 weeks, people who received inactive medication lost only 3.1% of their body weight, while those who received a Mounjaro dosage lost up to almost 21%, depending on the dosage received.

Although Mounjaro is used off-label for weight loss, Zepbound (tirzepatide), also manufactured by Eli Lilly, was FDA approved in November 2023 specifically for weight management.

Can you regain weight after using Mounjaro?

“Unfortunately, weight regain after stopping tirzepatide is very common,” says Dr. Nathanson. While tirzepatide can combat the physiologic mechanisms that cause weight gain, these effects cease when the medicine is stopped. In other words, weight loss from Mounjaro may not be permanent. Developing healthy dietary and exercise modifications can help sustain tirzepatide’s results to a degree, but weight gain may still occur after stopping the medication. 

In a 2023 trial investigating the effects of sustained treatment with tirzepatide, individuals who remained on the medication after the initial 36-week period (after an average weight loss of 20.9% after the first 36 weeks) had an additional 5.5% weight reduction after another 52 weeks. On the flip side, patients who were given a placebo rather than tirzepatide (despite maintaining a healthy diet and exercise regimen) experienced a 14% weight gain. “The data suggests that suddenly stopping the medication after achieving a goal weight is a recipe for weight regain,” Dr. Nathanson says. 

Many providers and patients have been experimenting with protocols for weight management or maintenance, such as decreasing dosage or spacing out intervals between doses, but research and trials are needed to support this.

Ensuring the preservation of muscle mass through adequate protein intake and resistance training may also play a role in keeping weight off after Mounjaro. “We know that maintaining adequate muscle mass affects the metabolism in a way that prevents weight regain,” Dr. Nathanson explains. “Therefore, many of us are optimistic that there may be a pathway for successfully maintaining weight after eventually stopping the medication.” 

Does Mounjaro work better than Ozempic?

For managing blood glucose levels, a 2021 trial found tirzepatide to be slightly more effective than semaglutide (the active ingredient in brand-name Ozempic).

Although no clinical trials have been conducted directly comparing the weight loss effects of Monjauro and Ozempic as a primary outcome in people who do not have diabetes, separate studies investigating the effectiveness of semaglutide and tirzepatide for weight loss in people with diabetes have shown that tirzepatide (Mounjaro or Zepbound) may be more effective.

According to a 2021 study, individuals who took a once-weekly injection of Wegovy (the version of semaglutide FDA-approved for weight loss) experienced an average weight loss of 15% after 68 weeks. Results from the SURMOUNT-1 study showed that individuals who took a comparable dose of tirzepatide at 72 weeks lost an average of 20.9% of their body weight. 

According to Dr. Nathanson, both medications have a similar side effect profile, with gastrointestinal side effects like abdominal pain or constipation being the most common.

RELATED: Mounjaro vs. Ozempic

The bottom line: Expect results in several weeks

Mounjaro starts working immediately, but it may take several weeks for noticeable changes in your blood sugar. For weight loss, Mounjaro can affect appetite immediately. Weight loss typically starts to occur within the first four weeks, with significant weight loss occurring over a period of 72 weeks. 

Before taking Mounjaro to lose weight, consult a healthcare professional for medical advice and to ensure it’s the right medication for you.