How to dose retatrutide

Retatrutide (Retatrutide) is dosed once a week by means of a subcutaneous injection. The Body Pharm dosing protocol is a ladder of three steps of four weeks each: 2 mg per week for 4 weeks, then 4 mg per week for 4 weeks and then 8 mg per week for 4 weeks, with a maximum dose of 12 mg per week. The pen dials each one of those doses directly in milligrams.

The maximum dose is 12 mg per week. The pen dials that value as well as the others.

That table is the protocol for the product. The ladder that the published clinical trials used is a different thing: it is dealt with further down, under "The trial ladder is not the product protocol", and it should not be read as the dosing schedule for this pen.

In 2026 retatrutide remains an investigational product without marketing authorisation; nothing that this guide contains constitutes medical advice.

Retatrutide dosing: the essentials

• Start with 2 mg a week for four weeks; move up a step only if all four tolerance criteria are met (nausea subsides within 48 hours, no vomiting in 14 days, appetite suppression present but with an intake above around 1200 kcal a day, injection-site reactions fully resolved).
• Hold each dose step for the full four weeks regardless of how well it is tolerated; stepping up too early is the most frequent cause of gastrointestinal intolerance.
• The last step of the ladder is 8 mg per week and the maximum dose is 12 mg per week. In the phase 2 trial, the 12 mg arm reached a mean weight loss of 24.2% at 48 weeks, with clearly more intense nausea and constipation [1].
• The triple-receptor mechanism of retatrutide (GIP/GLP-1/glucagon) demands more titration discipline than semaglutide or tirzepatide, because the activity on glucagon widens the window of adverse effects at each step.
• Store the pens between 2 and 8 °C and protected from light. Unopened, they keep for 24 months; once they have been punctured for the first time, use them within a period of 12 months. Let them reach room temperature before you inject and do not freeze them.

What retatrutide is and how it works

Retatrutide is a single-molecule triple agonist that activates the receptors for GIP (glucose-dependent insulinotropic polypeptide), GLP-1 (glucagon-like peptide-1) and glucagon [1][2]. Eli Lilly developed it and described it for the first time in the phase 2 obesity study by Jastreboff et al., published in the New England Journal of Medicine in June 2023 [1].

The glucagon arm is what sets it apart pharmacologically from semaglutide (GLP-1 only) and from tirzepatide (GIP/GLP-1). The activity on the glucagon receptor drives the mobilisation of hepatic lipids and a moderate thermogenic component, in addition to the effects on appetite and gastric emptying that the incretin arms provide [1].

When it comes to dosing, that third receptor is relevant for two reasons. The efficacy ceiling is higher (around 24.2% mean weight loss at 48 weeks in the 12 mg arm) [1]. In addition, the titration window has to take account of effects derived from glucagon, such as the increase in heart rate and hepatic flux, alongside the already familiar incretin gastrointestinal profile, which is why the increments are staggered more conservatively than in a tirzepatide ladder [1]. Some researchers worry that the added receptor complexity will be tolerated less well than in the dual agonists; the phase 2 data suggest that holding each step for four weeks with discipline reduces that risk considerably.

See the comparison Retatrutide vs semaglutide vs tirzepatide to contrast the mechanisms and Tirzepatide dosing for the parallel with the dual agonist.

In 2026 retatrutide does not hold marketing authorisation and is regarded as an investigational product [3][4].

Retatrutide dosing schedule at a glance (2026)

The Body Pharm dosing protocol is a ladder of three steps of four weeks each, with a maximum dose of 12 mg per week. Each step is held for the full four weeks before moving up [5].

Ahead of that protocol there is room for an optional preliminary step. The canonical titration in research is a ladder of four steps: an optional microdose of 0.5 mg to check sensitivity, a standard start of 2 mg, a step to 4 mg and maintenance at 8 mg, holding each step for four weeks before moving up.

The microdose is an optional preliminary step, not a fourth step of the protocol. If it is used, it takes up four weeks of its own ahead of the start and shifts the rest of the ladder along: 2 mg in weeks 5–8, 4 mg in weeks 9–12 and 8 mg from week 13 onwards. The weeks shown in the table are those of someone who does not use it and who starts directly at 2 mg.

The maximum dose of the protocol is 12 mg per week. The pen dials every dose on the ladder, including the 12 mg one, directly in milligrams: the solution comes ready to use and there is nothing to prepare and no volumes to calculate.

Hold each step for the full four weeks, regardless of how well it happens to be tolerated. Stepping up too early is the most frequent cause of gastrointestinal intolerance in research series [1].

The trial ladder is not the product protocol

The published clinical trials used a ladder of their own, and it is not the schedule for this pen. The phase 3 TRIUMPH programme escalated 2 → 4 → 6 → 9 → 12 mg weekly [3]; in TRIUMPH-1 (2026), the 4, 9 and 12 mg arms lost 17.6%, 23.7% and 25.0% mean weight at 80 weeks against 3.9% with placebo [8]. The phase 2 study by Jastreboff et al. worked with arms of 1 mg, 2 mg, 4 mg, 8 mg and 12 mg; the 12 mg cohort reached around 24.2% mean weight loss at 48 weeks, at the cost of a greater burden of nausea and constipation [1].

Those sequences are published data from the trials and they are cited here as a bibliographic reference. The dosing protocol for the product is the one in the table above: 2 mg → 4 mg → 8 mg in steps of four weeks, with a maximum of 12 mg per week.

The justification for this ladder as against that of a dual agonist is in Retatrutide vs semaglutide vs tirzepatide; the parallel GIP/GLP-1 schedule is in Tirzepatide dosing.

Why start at 2 mg (or 0.5 mg)

Rapid escalation to therapeutic levels brings on vagally mediated nausea that is intense enough to interrupt a research series. Agonism of the GLP-1 receptor in the area postrema and in the nucleus of the solitary tract drives the emetic response. Starting low gives the central receptors time to downregulate before exposure increases.

The data from phase 2 back this up with figures. Jastreboff et al. (NEJM, 2023) described gastrointestinal adverse events as the dominant tolerance signal in all the retatrutide arms, and reported that they increased markedly with the size of each escalation step [1]. Protocols with a slower ramp-up (starting at 2 mg instead of going directly to 4 mg) clearly reduced the gastrointestinal burden during the escalation [1]. Each of the four-week holds at 2 mg, 4 mg and 8 mg keeps the receptor population ahead of the dose. Anyone who fears that a slow start will delay efficacy should bear in mind that the 8 mg step is reached in week 9 all the same, and that the gastrointestinal tolerance built up during weeks 1–8 translates into better adherence and longer series.

The start of the protocol is 2 mg per week: the 0.5 mg microdose is an optional preliminary step that comes before it, not an alternative to that start and not a fourth step of the ladder. Anyone arriving from a poorly tolerated semaglutide or tirzepatide titration will find in the next section when that preliminary step makes sense.

When the 0.5 mg microdose makes sense

The 0.5 mg preliminary step does not form part of the Jastreboff protocol. It is a convention inherited from practice with semaglutide and tirzepatide for researchers with documented gastrointestinal sensitivity to incretin agonists, especially for those who tolerated 0.25 mg of semaglutide poorly or needed antiemetic support during a previous tirzepatide titration [1]. The logic is identical: a sub-therapeutic conditioning dose allows the gut and the central receptors to acclimatise before the standard ladder gets under way.

For the comparison, see Retatrutide vs semaglutide vs tirzepatide; the parallel escalation logic is in Tirzepatide dosing.

The four-week rule: when to move up

You only move on to the next step after four consecutive weeks at the current dose and if all four of the tolerance criteria are met; otherwise, the dose is held or dropped back by one step. The four-week minimum is the receptor-adaptation window taken directly from the phase 2 titration arms of Jastreboff et al. 2023 [1], and not an arbitrary convention of the calendar.

The checklist that follows is the protocol synthesis made by Body Pharm, derived from the phase 2 scheme but not validated by it. Treat it as expert opinion, not as criteria confirmed by the study.

If all four of the criteria are met in week 4, you move up. If one of them fails, the dose is held for two more weeks at the same level and reassessed in week 6. If the same criterion fails again in week 6, the protocol drops back one step (for example, from 4 mg to 2 mg) for four weeks before attempting the step up again. Failing repeatedly at the same threshold means that the receptor population has not yet caught up with the agonist load; forcing the ladder generates exactly the gastrointestinal burden that the slow-titration arm was designed to avoid [1].

The four-week minimum is not a four-week maximum

Several frameworks based on practice from 2024–2025 describe researchers who remain for between 8 and 12 weeks at 4 mg before moving up to 8 mg. The reasoning: a longer stay in the middle of the ladder improves long-term tolerance and reduces drop-outs at the step from 8 to 12 mg. That is expert opinion, not study evidence, but the pharmacological logic is coherent. In phase 2, the jump from 4 to 8 mg was the point at which the gastrointestinal events became more acute [1]. A longer plateau at 4 mg gives both the central and the peripheral receptors more time to downregulate before the next step.

For the parallels with the step sequence of a dual agonist, the Tirzepatide dosing guide uses the same hold-or-step-up logic, in the context of the narrower adverse-effect window of tirzepatide.

Subcutaneous injection: step by step

The Body Pharm multi-dose pen administers retatrutide by the subcutaneous route with the solution already ready to use. The technique described: dial the dose on the dose dial, inject at a 90° angle over 5–10 seconds into the abdomen or the thigh and rotate the sites following a fixed grid. The procedure that follows assumes a 32 mg or a 64 mg pen stored between 2 and 8 °C and protected from light.

Site rotation

In the protocols described, the same 2 cm area is not used again within a period of seven days. A common pattern is a grid of four quadrants per half of the abdomen (eight areas in total) worked through clockwise. That avoids the lipohypertrophy and the uneven absorption that appear in people who use insulin with rotation patterns that are too tight. The same principle governs insulin injection technique.

The pen format

Retatrutide is supplied in a pre-filled multi-dose pen with a 3 ml cartridge. The dose dial shows the dose directly in milligrams, so the only figure that has to be recorded per administration is the one that appears in the dose window.

The two presentations differ from each other in their total peptide content: the 32 mg pen carries 32 mg and the 64 mg pen carries 64 mg. The dose is read in mg on the dose dial in both cases; the 64 mg pen simply lasts longer before it runs out.

Discard any pen whose solution turns cloudy, becomes discoloured or shows particles.

Managing adverse effects at each dose step

The intensity of adverse effects follows the titration ladder almost linearly: mild and self-limiting at 2 mg, transient but noticeable at 4 mg, clinically relevant from 8 mg upwards. In the protocols described, the response is organised step by step rather than by reacting to the symptoms as they appear.

The 2 mg step

Expect a mild suppression of appetite in the 48 hours after the first injection. In a minority of cases there is transient nausea on the day of the injection, which subsides within 24 hours. At this step there is no need for any symptomatic intervention. If nausea at 2 mg lasts longer than 48 hours, hold the step instead of moving up: a bad series at 2 mg foreshadows a worse one at 4 mg.

The 4 mg step

With the first one or two doses of 4 mg, nausea usually lasts between 24 and 48 hours after the injection. The pragmatic convention in the GLP-1 research community is to inject in the evening, so that the peak of nausea coincides with sleep. That is the experience of users, not evidence from studies. Keep fluid intake at a minimum of 2 litres a day, since the reduction in appetite usually brings the consumption of water down as well.

The 8 mg step

Nausea and tiredness are concentrated in weeks 1–2 at 8 mg. After that, constipation goes on to become the dominant complaint. Jastreboff et al. (NEJM, 2023) [1] described constipation in around a quarter of the participants in the higher-dose arms, although the exact breakdowns by cohort are not published in the open-access summaries. Prevent it with 25–30 g of fibre a day and a steady fluid intake, before the symptoms appear. Reactive management is consistently slower than prophylaxis is at this step. If constipation persists for more than seven days despite fibre and fluids, delay the next step.

The 12 mg step

Here the gastrointestinal burden is at its maximum, and it was the main limit on tolerance in the phase 2 data from 2023 [1]. Many researchers stay at 8 mg precisely for that reason. According to reports, going above 8 mg brings with it a re-acclimatisation window of 2–3 weeks.

At every step

Mild redness or bruising at the injection site is normal and it subsides within 48 hours. Persistent subcutaneous nodules point to poor rotation; review the plan of sites first before attributing the effect to the substance. Hypoglycaemia is not a primary risk, because the incretin effect of retatrutide is dependent on glucose. Anyone combining it with insulin sensitisers, sulfonylureas or other incretins should take additive effects into account. For the comparative adverse-effect profiles against the dual agonist, the Tirzepatide dosing guide shows where the two diverge most at the equivalent 8 mg step.

Retatrutide vs semaglutide vs tirzepatide: dosing differences

Retatrutide demands more titration discipline than its dual and mono-agonist comparators, because agonism of the glucagon receptor widens the window of adverse effects at each step. The table puts the three ladders side by side with one another; all of them use steps of four weeks as the standard escalation rhythm.

Anyone who is used to the narrow dose range of semaglutide underestimates the gastrointestinal burden of the step from 8 to 12 mg of retatrutide. The 2023 phase 2 data showed the steepest drop in tolerance at that transition [1]. Tirzepatide sits between the two of them in the intensity of its adverse effects, but it does not have the glucagon-driven hepatic and energy-expenditure effects that make retatrutide interesting. The full comparison of mechanism and outcomes is in Retatrutide vs semaglutide vs tirzepatide; the protocol for the dual agonist on its own is in the Tirzepatide dosing guide, in the same ladder format.

Storage, handling and shelf life of the pen

Keep retatrutide pens refrigerated between 2 and 8 °C, protected from light in their original packaging, and never freeze them. Freezing destabilises the peptide in an irreversible way. Ambient light accelerates degradation even when the cartridge appears to be intact.

Unopened, the pen keeps for 24 months. Once it has been punctured for the first time, use it within a period of 12 months. If you have no fridge available, the pen withstands up to 30 days at room temperature, with a maximum of 30 °C. Let it reach room temperature before you inject and do not freeze it.

Checks before each injection:

• Examine the cartridge against a white background. Discard it if the solution is cloudy or discoloured or if it shows visible particles.
• Take the pen out of the fridge 20–30 minutes before dosing, so that the solution reaches around 20–25 °C. Cold injections sting and can affect the consistency of the dose in pens that are driven by a spring.
• For travel, use an insulated bag with a cold pack separated from the pen by a layer of fabric. Direct contact with the ice risks local freezing of the cartridge.

If a pen has accumulated more than 30 days outside the fridge, or has at any point exceeded 30 °C, it is regarded as compromised and not reusable.

Frequently Asked Questions

Five questions come up again and again in the research forums and in correspondence. The answers draw on the protocol above and on the 2023 phase 2 dataset.

Can the weekly dose be split into two injections?

No. Retatrutide is a peptide for weekly subcutaneous administration with a half-life of several days that maintains a stable plasma exposure over a 7-day interval [1]. Splitting 8 mg into two injections of 4 mg lowers the peak concentration without appreciably reducing the burden of gastrointestinal adverse effects. Nausea follows the cumulative weekly exposure, and not the peak. The phase 2 protocol used single weekly injections throughout [1].

What happens if a scheduled dose is missed?

The missed dose is injected as soon as it is noticed, provided that fewer than 4 days have passed since the scheduled day. If the gap is any longer, the dose for that week is skipped entirely and you continue on the next scheduled day with the same dose. Doubling the dose in order to make up for it is not recommended, because the resulting peak exposure reliably brings on nausea and vomiting at maintenance doses of 8 mg or more.

Is 8 mg the maximum dose, or does it make sense to go up to 12 mg?

8 mg is the last step of the titration ladder; the maximum dose of the protocol is 12 mg per week, and the pen dials that value just like the previous ones. In the phase 2 obesity study by Jastreboff et al. 2023, 12 mg was the highest dose evaluated and achieved around 24% mean weight loss at 48 weeks, although with a considerably greater burden of gastrointestinal adverse effects than at 8 mg [1]. The phase 3 TRIUMPH programme kept 12 mg as its highest arm with a ladder of its own of 2 → 4 → 6 → 9 → 12 mg, which is the ladder of the trial and not the one for the product [3]; in TRIUMPH-1 (2026) that arm reached 25.0% mean weight loss at 80 weeks [8]. For the comparison, see Retatrutide vs semaglutide vs tirzepatide.

Can retatrutide be combined with other peptides?

The 2023 phase 2 dataset and all the practice-based frameworks from 2024–2025 treat retatrutide exclusively as monotherapy [1]. No peer-reviewed work addresses its simultaneous administration with semaglutide, tirzepatide or other incretin agonists. Combining incretin agonists with one another produces additive nausea, delayed gastric emptying and constipation. Anyone who is developing a tirzepatide protocol in parallel should read Tirzepatide dosing and treat any overlaps as contraindicated.

How many weeks does a pen last at the maintenance dose?

A 32 mg pen covers four weeks of dosing at 8 mg per week. A 64 mg pen covers eight weeks of dosing at the same dose. At 4 mg per week (early titration), the 32 mg pen lasts eight weeks and the 64 mg pen sixteen. In all of those cases the cartridge is used up within the 12 months of use that the pen allows from the first puncture.

Research notice and legal status

In 2026 retatrutide is not authorised as a medicine for use in humans [4]. It remains an investigational triple agonist whose main evidence base is the phase 2 obesity study by Jastreboff et al. (NEJM, 2023) [1]. The first phase 3 results from the TRIUMPH programme (TRIUMPH-1, NEJM 2026) were published in 2026 and confirm that signal, without changing its regulatory status [3][8].

All of the Body Pharm retatrutide pens are sold exclusively for research and analytical purposes and are not intended for human consumption. This guide summarises the dosing described in the published literature and does not constitute medical advice. Researchers should consult the primary publication in NEJM 2023 and the relevant institutional ethical frameworks; for the comparison, see Retatrutide vs semaglutide vs tirzepatide and Tirzepatide dosing.

Next steps

It is recommended that you read the phase 2 protocol by Jastreboff et al. (NEJM, 2023) before starting the titration. The usual practice is to keep a four-week injection log with columns for date, time, site, dose, batch of the pen and tolerance notes. Anyone with previous experience of semaglutide or tirzepatide can consult the comparison Retatrutide vs semaglutide vs tirzepatide to see where the triple agonist departs most from them. Starting at 2 mg and holding each step for four weeks with consistency is the protocol that is described; stepping up too early is the most frequent cause of drop-out through gastrointestinal intolerance.

Sources

1. Jastreboff AM, et al. Triple-Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial. N Engl J Med. 2023. pubmed.ncbi.nlm.nih.gov/37366315
2. Rosenstock J, et al. Retatrutide, a GIP, GLP-1 and glucagon receptor agonist, for people with type 2 diabetes: a randomised, double-blind, placebo and active-controlled, parallel-group, phase 2 trial. Lancet. 2023. pubmed.ncbi.nlm.nih.gov/37385280
3. ClinicalTrials.gov, registry of retatrutide trials (phase 3 TRIUMPH programme), consulted in October 2026. clinicaltrials.gov
4. EMA, list of medicines for human use authorised in the EU (EPAR). ema.europa.eu — consulted in October 2026 for the regulatory status of the compound.
5. Body Pharm dosing protocol for the retatrutide pen (2 → 4 → 8 mg weekly, maximum 12 mg) (supplier specification, reviewed in October 2026). Supplier research protocol, not a peer-reviewed study.
6. Wilding JPH, et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1). N Engl J Med. 2021. pubmed.ncbi.nlm.nih.gov/33567185
7. Jastreboff AM, et al. Tirzepatide Once Weekly for the Treatment of Obesity (SURMOUNT-1). N Engl J Med. 2022. pubmed.ncbi.nlm.nih.gov/35658024
8. Jastreboff AM, et al. Retatrutide, a Triple Hormone Receptor Agonist, for Treatment of Obesity (TRIUMPH-1). N Engl J Med. 2026. pubmed.ncbi.nlm.nih.gov/42814954