How to lose fat on Mounjaro or Zepbound

Mounjaro and Zepbound are the same drug, tirzepatide. Zepbound is approved for weight loss and sleep apnea, and Mounjaro for type 2 diabetes. In the main weight loss trial, adults on the top weekly dose lost 20.9% of their weight in 72 weeks, against 3.1% on placebo. About a quarter of what they lost was lean mass, so protein and lifting matter.

Mounjaro or Zepbound: what’s the difference?

Zepbound is for adults with a BMI of 30 or more, or 27 or more with a weight-related condition like high blood pressure, type 2 diabetes or high cholesterol.

How tirzepatide works

Tirzepatide acts on the receptors for two gut hormones, GLP-1 and GIP. Both receptors sit in the parts of the brain that control appetite, so people feel less hungry and eat less. It also slows how fast the stomach empties, most of all after the first dose. It lasts about five days in the blood, which is why it’s a weekly shot.

Picture two hands on the hunger dial instead of one.

What the trials found

At least 85% of people on any dose lost 5% or more of their weight, and on 15 mg, 57% lost 20% or more. Against semaglutide head to head, in SURMOUNT-5 (751 adults, 72 weeks), tirzepatide took off 20.2% and semaglutide 13.7%, and waists shrank by about 7.2 inches (18.4 cm) against 5.1 (13.0 cm).

How much of the loss is muscle?

In SURMOUNT-1, 160 people had DEXA scans. On tirzepatide they lost 33.9% of their fat and 10.9% of their lean mass, about 35 pounds (15.9 kg) of fat and 12 pounds (5.6 kg) of lean mass. That’s 74% fat and 26% lean. The placebo group, who lost weight through diet and exercise advice, split 75% and 25%. So the drug didn’t take an unusual share of lean mass. It took a lot of weight, and the usual share of that was lean.

Lean mass counts water, organs and skin as well as muscle, so not all of the 12 pounds was muscle.

How to keep muscle on it

• Protein first. The 2025 US advisory suggests about 1.5 g per kg of lean mass, or 80 to 120 grams a day. Make every meal, however small, start with protein. Find your range.
• Lift. The advisory suggests strength training at least three times a week and at least 150 minutes of moderate cardio.
• Mind the pace. In athletes, faster loss kept less lean mass. If you’re losing much more than 1% of your weight a week, raise it with your prescriber.
• Track strength and waist. The fat or muscle calculator splits two weigh-ins a month apart.

What happens when you stop

In SURMOUNT-4, 783 adults took tirzepatide for 36 weeks and lost 20.9% of their weight. Then half were switched to a placebo. Over the next year they regained 14% of their weight, while those who stayed on the drug lost another 5.5%. Over the whole 88 weeks, the group that stayed on lost 25.3% and the group that stopped 9.9%. Build the habits that hold your weight while you’re on it, and plan any stop with your doctor.

The side effects that matter

The label also warns of severe stomach reactions, kidney injury from losing fluid, gallbladder problems, pancreatitis, allergic reactions and low blood sugar with insulin or some diabetes pills. It carries a boxed warning because tirzepatide caused thyroid tumors in rats, so people with a personal or family history of medullary thyroid cancer or MEN 2 shouldn’t take it. It shouldn’t be combined with Mounjaro or another GLP-1 drug. Tell your surgeon before any procedure that needs anesthesia. The label says it may harm an unborn baby, and that it can make birth control pills work less well for 4 weeks after starting and after each dose step, so ask your doctor about a backup or a non-pill method.