Retatrutide Dosing Protocol: Starting Dose, Titration Schedule & What to Expect

What Is Retatrutide?

Retatrutide (LY3437943) is a triple GIP/GLP-1/glucagon receptor agonist developed by Eli Lilly. Where tirzepatide activates two receptors (GIP and GLP-1), Retatrutide adds glucagon receptor agonism — a third mechanism that further amplifies fat oxidation and metabolic rate. This triple-action profile produced the most dramatic weight loss results in any Phase 2 GLP-1 trial to date: participants lost an average of 24.2% of body weight at 48 weeks on the highest dose, compared to roughly 20% for tirzepatide and 15% for semaglutide in their respective trials.

As of 2026, Retatrutide has not received FDA approval and is not commercially available as a prescription medication. It is available through research chemical suppliers and some compounding pharmacies as a research compound. Anyone using it is doing so outside approved clinical parameters and should do so only with physician oversight.

This guide is for informational purposes and reflects the dosing approach used in Eli Lilly's Phase 2 trial (NCT04881357) and the community protocols that have developed around it. It is not medical advice.

Starting Dose: Why Low and Slow Matters

The most common mistake new Retatrutide users make is starting too high. The compound is significantly more potent than most people expect, and GI side effects — nausea, vomiting, diarrhea — during the ramp-up phase are the primary reason people abandon the protocol before reaching therapeutic doses.

The clinical trial starting dose was 0.5 mg once weekly for the first four weeks. This is low enough that most people experience minimal side effects while the body adjusts to GLP-1 receptor agonism and the associated changes in gastric emptying rate. Some protocols in the community begin at 0.25 mg for 2 weeks before stepping up to 0.5 mg — this ultra-slow start is particularly valuable for people who have previously experienced severe nausea on GLP-1 agonists.

A consistent weekly injection day matters from the start. Choose a day — many people prefer Sunday or Monday — and stick to it. Consistency reduces the variability in plasma levels that can exacerbate side effects, and it makes dose tracking straightforward. Once you commit to your injection day, your labeled vials in an organized case become your record-keeping system: you always know exactly which vial you're on and how much remains.

Titration Schedule: The Standard Ramp

The Phase 2 trial used a structured titration that increased dose every 4 weeks. This 4-week hold at each dose level is important — it gives your GI system time to adapt before you add more agonist load. Rushing titration is the most reliable way to end up nauseated and vomiting for a week.

A standard community protocol based on the trial design:

• Weeks 1–4: 0.5 mg once weekly
• Weeks 5–8: 1 mg once weekly
• Weeks 9–12: 2 mg once weekly
• Weeks 13–16: 4 mg once weekly
• Weeks 17–20: 6 mg once weekly
• Weeks 21+: 8 mg once weekly (maintenance dose for most users; some trial participants went to 12 mg)

If you experience significant side effects at any dose level, hold that dose for an additional 4 weeks rather than escalating. There is no competitive advantage to reaching higher doses faster — the weight loss is occurring at every dose level in the titration, and GI tolerance is the binding constraint on your protocol's success.

Reconstitution Math: Calculating mg per Unit

Most Retatrutide arrives lyophilized in vials of 5 mg or 10 mg. You reconstitute with bacteriostatic water (BAC water) — see our full reconstitution guide for technique — and then draw specific volumes with an insulin syringe to hit your target dose.

The standard reconstitution formula:

• Add 1 mL (100 units on a U-100 insulin syringe) of BAC water to a 5 mg vial → concentration = 5 mg/mL = 0.05 mg per unit
• Add 2 mL (200 units) of BAC water to a 10 mg vial → same concentration: 5 mg/mL = 0.05 mg per unit

Dose calculations at 5 mg/mL concentration:

• 0.5 mg dose: 10 units on the syringe
• 1 mg dose: 20 units
• 2 mg dose: 40 units
• 4 mg dose: 80 units
• 6 mg dose: 120 units (requires drawing twice from a U-100 syringe, or using a 0.5 mL syringe)
• 8 mg dose: 160 units (use a larger syringe or adjust reconstitution volume)

For higher doses, consider reconstituting with less BAC water to increase concentration. Adding 0.5 mL to a 5 mg vial gives 10 mg/mL (0.1 mg/unit), cutting all volumes in half. Always recalculate your draw volume if you change your reconstitution volume.

Injection Timing, Site, and Technique

Retatrutide is administered subcutaneously — injected into the fat layer just below the skin, not into muscle. Common injection sites are the abdomen (2 inches away from the navel), outer thigh, and back of the upper arm. Rotate sites each week to prevent lipohypertrophy (fatty tissue buildup) at a single site.

Technique matters more than most people think. Key points:

• Allow the reconstituted vial to reach room temperature for 10–15 minutes before injecting. Cold solutions sting more and absorb differently.
• Clean the injection site with an alcohol swab and allow it to dry completely before inserting the needle. Injecting through wet alcohol can sting.
• Pinch the skin gently, insert at a 45° angle for thin individuals or 90° for those with more subcutaneous fat, and inject slowly — 10–15 seconds for the full volume.
• Do not rub the injection site after injecting. Let the solution absorb naturally.

Weekly injection timing can be adjusted based on your lifestyle. Many people inject Friday evening so that any mild side effects (reduced appetite, slight nausea) occur over a weekend rather than during a workday. Others prefer morning injections. There is no clinically meaningful difference in outcome — pick the time that you'll adhere to consistently.

Managing Side Effects During Ramp-Up

GI side effects are the most common reason people abandon GLP-1 protocols. With Retatrutide, the triple-agonist mechanism means the GI adaptation window can be more pronounced than with single or dual agonists. Tactics that help:

• Eat smaller meals. GLP-1 agonists slow gastric emptying. Eating the same meal volumes you ate before starting will reliably cause nausea. Reduce meal size proactively, especially in the first 2 weeks at each new dose level.
• Avoid fatty, spicy, and acidic foods during the first week at each new dose. These are the most common nausea triggers on GLP-1 protocols.
• Stay hydrated. GLP-1-induced appetite suppression can cause people to under-drink as well as under-eat. Aim for consistent water intake even when you don't feel thirsty.
• OTC anti-nausea options. Ginger chews, Vitamin B6, or Pepto-Bismol can manage mild nausea. If nausea is severe enough to prevent eating or drinking, hold your current dose and consult a physician before escalating.
• Don't escalate through significant side effects. A rough 2–3 days after a dose increase that resolves is normal. Persistent vomiting or inability to eat is a signal to hold and possibly back down a dose level.

Storing Reconstituted Retatrutide: What You Need

Reconstituted Retatrutide must be refrigerated at 2–8°C and protected from light. At these temperatures, stability is generally maintained for 4–6 weeks — which aligns well with typical vial consumption at standard doses. Do not freeze reconstituted vials; freeze-thaw cycles damage peptide structure and reduce potency.

At higher doses (6–8 mg/week), a single 10 mg vial lasts only 1–2 weeks. This means active rotation through multiple vials. An organized storage system isn't a luxury at this point — it's a clinical necessity. Confusing which vial was reconstituted when, or accidentally drawing from a vial that has been sitting at room temperature for days, can mean under-dosing or using degraded compound.

A dedicated peptide case in your refrigerator, with labeled vials showing reconstitution date and concentration, eliminates this confusion entirely. Label each vial when you reconstitute it: compound name, concentration, date, and your initials if others use the fridge. This five-second habit protects weeks of expensive compound and keeps your protocol on track. Check the full Retatrutide storage guide for complete shelf-life and handling details.