What is the difference between semaglutide, tirzepatide and retatrutide?

The GLP-1 conversation has collapsed into a single word: Ozempic. That is a brand name. The molecules underneath it are a small, evolving family of drugs, and they do not behave the same way.

This piece is a plain explainer for men asking about "the weight loss shot." No marketing copy. No Reddit folklore. Just what the data says and how a clinician thinks about which option fits which patient.

What is a GLP-1 medication?

A GLP-1 (glucagon-like peptide-1) medication mimics gut hormones your body already produces after eating. Most are weekly injections. Some are daily pills. The effects are:

• Slowed gastric emptying, which makes you feel full longer.
• Better insulin response to food, which smooths post-meal blood sugar.
• Reduced appetite signaling, both centrally and peripherally.

The newer drugs in the class do more than just GLP-1 mimicry. They layer on other gut hormones with their own effects. Same family, different members.

The three GLP-1 medications men ask about

GLP-1 alone (semaglutide) is appetite suppression plus slowed gastric emptying plus better post-meal insulin response. Adding GIP (tirzepatide) layers in better insulin sensitivity and cleaner body-composition change in trials. Retatrutide adds glucagon agonism. It is investigational: it is not FDA approved, it is only given in clinical trials, and nobody can prescribe it to you today.

How do GLP-1s work for weight loss?

The mechanism is mostly upstream of "less food." GLP-1 receptor activation reduces hunger signals in the hypothalamus and brainstem. Slowed gastric emptying means each meal feels larger. The combined effect is a sustained, low-grade calorie reduction that most patients describe as "I stopped thinking about food" rather than "I am dieting."

In trials, the average weight loss over 68 weeks at maximum dose:

• Semaglutide 2.4 mg/week: about 15% of body weight.
• Tirzepatide 15 mg/week: about 21% of body weight. Those numbers come from trial patients with obesity. Results in other groups can differ. Retatrutide has earlier-phase trial data only. It is investigational and not available, so it is not in this list.

The muscle-loss problem

The headline number in every trial is total weight lost. The number that matters for a 38-year-old man is what he lost.

GLP-1s reduce calorie intake. A meaningful share of weight lost on any sustained calorie deficit is lean mass. That is gravity, not the drug's fault. But you can stack the deck:

• Protein: 1.6 g/kg of bodyweight as a floor, not a ceiling.
• Resistance training: three sessions a week, progressive. Cardio is fine but not the lever.
• Loss rate: faster weight loss equals more lean mass given up. About 0.5%/week of bodyweight is the upper edge of "mostly fat."

This is covered in depth in Muscle loss on GLP-1s. The short version: the drug does not decide what you lose. The protocol does.

What the clinician actually checks

Before any GLP-1, a careful clinician looks at the panel, not only the BMI.

Markers worth checking before a GLP-1 starts

• Fasting insulin is the cleanest early signal of insulin resistance, often before A1c moves.
• ApoB is the lipid number that correlates with cardiovascular event risk better than LDL alone.
• Visceral fat (DEXA preferred, BIA acceptable) distinguishes the man who needs a GLP-1 from the man who needs a different lever entirely.
• Liver enzymes plus GGT flag fatty liver, which changes the conversation.
• Thyroid panel rules out endocrine causes of weight gain masquerading as metabolic disease.
• Testosterone (free plus total) matters because low T flattens the calorie-deficit response. You fix the upstream problem first.

Side effects that actually matter

Early-week nausea is real and well-documented. It is not the side effect that should drive your decision.

The conversations worth having:

1. Gastroparesis risk. Most men tolerate slowed gastric emptying. A subset develops more persistent symptoms. Reversible on cessation. Not zero risk.
2. Gallbladder issues. Meaningful absolute risk increase with rapid weight loss. Higher with faster loss.
3. Sarcopenia (muscle loss). Covered above. Not on the side-effect leaflet because it is a function of how you use the drug, not the drug itself.
4. Mental health signals. Early reports flagged mood effects in some patients. Later analyses have been more reassuring. Baseline mood is worth checking, and worth asking about at follow-ups.

How long does it take to work?

Appetite usually shifts within the first week or two. Weight movement is measurable by week 4. Body composition change (fat down, muscle kept) takes 3 to 6 months when the plan is built right.

Who should not take a GLP-1?

GLP-1s are generally not a fit for:

• Personal or family history of medullary thyroid carcinoma or MEN-2.
• Active or prior pancreatitis.
• Pregnancy or planned pregnancy in the next 8 weeks.
• Severe gastroparesis or gastric outlet obstruction.
• BMI below 22 without a metabolic indication, in most cases.

This is not the full list. A clinician will go through your history before any prescription is written.

Where this lands

A GLP-1 plan that works tends to have four parts:

• Right molecule for the metabolic signature, not for the marketing.
• Right dose, set and adjusted by a clinician.
• Strength training and protein as part of the plan, not optional extras.
• Labs rechecked over time, including ApoB, fasting insulin, lean mass and liver enzymes. The medicine is one variable in a system.

GLP-1s are one of the highest-leverage tools in this generation of men's medicine. They are also the tool most likely to be used wrong. The difference is upstream of the drug.

If you are considering starting, start with a panel, not a prescription.