Retatrutide (Reta) Side Effects: What the Trials Show

Retatrutide is Eli Lilly's experimental once-weekly injection that acts on three hormone receptors: GIP, GLP-1 and glucagon. Online it is mostly called "reta". It is not approved by the FDA for any use. Everything below comes from the clinical trials, where people were given drug made by Lilly under medical supervision. The retatrutide sold online as a "research peptide" is a different situation: the FDA treats it as an unapproved new drug and has sent warning letters to sellers ^[8], so its purity, dose and contents are unknown.

This page is a summary of published trial data, not medical advice.

The short answer

• Most side effects are digestive: nausea, diarrhea, vomiting and constipation. They rise with dose and are worst while the dose is being increased ^[1][3].
• Two signals stand out against placebo in the phase 3 TRIUMPH-2 trial: dysesthesia (altered skin sensation) and hypotension (low blood pressure) ^[3].
• Heart rate goes up in a dose-dependent way, peaking around week 24 and then falling ^[1].
• Stopping because of side effects was 4% at 4 mg, 12% at 9 mg and 8% at 12 mg in TRIUMPH-2, against 5% on placebo ^[3].

Digestive side effects, by dose

TRIUMPH-2 enrolled 1,152 adults with obesity and type 2 diabetes and is the first phase 3 retatrutide trial to publish exact side-effect rates ^[3]:

Side effect	Placebo	4 mg	9 mg	12 mg
Diarrhea	13%	27%	34%	34%
Nausea	8%	14%	21%	28%
Hypotension	<1%	1%	5%	6%
Dysesthesia	1%	4%	6%	7%
Stopped due to adverse events	5%	4%	12%	8%

TRIUMPH-1, in 2,339 adults with obesity but no diabetes, also reported that the most common adverse events were gastrointestinal ^[2]. Its full per-dose table is in the NEJM paper.

The phase 2 obesity trial found these effects were mostly mild to moderate. They were partly reduced by starting at 2 mg instead of 4 mg ^[1], which is why the phase 3 trials raised the dose step by step. In the phase 2 diabetes trial, digestive side effects ran from 13% at the lowest dose up to 50% in the group that escalated fastest ^[4].

Retatrutide also slows how fast the stomach empties ^[5]. That is part of why people feel full sooner, and part of why large or fatty meals can bring on nausea. The general advice in our GLP-1 side effects guide (smaller meals, less fat while the dose is rising, enough fluids) applies here too.

Dysesthesia: the side effect other GLP-1s don't show

Dysesthesia is tingling, burning, numbness or skin that feels sensitive to touch. In TRIUMPH-2 it affected 4–7% of people on retatrutide against 1% on placebo, and it rose with dose ^[3]. It is not a typical side effect of semaglutide or tirzepatide. The cause is not yet explained, and the published abstracts do not say how long it usually lasted.

Heart rate and blood pressure

In the phase 2 obesity trial, heart rate rose with dose, peaked at 24 weeks and declined after that ^[1]. Blood pressure moves the other way: TRIUMPH-2 recorded more hypotension on retatrutide, up to 6% at 12 mg ^[3]. Rapid weight loss, less eating and drinking, and fluid lost through diarrhea or vomiting can all lower blood pressure further. That matters most for anyone already taking blood-pressure medicine. A cardiovascular outcomes trial is part of the TRIUMPH program, and its results aren't published yet.

Does reta make you tired?

The published abstracts do not list fatigue among the main side effects of retatrutide. Tiredness is still commonly reported with GLP-1-type drugs, and the likely causes are ordinary ones:

• Eating much less. Appetite drops sharply, especially at higher doses, so total calories and protein can fall further than intended.
• Dehydration. Nausea, vomiting and diarrhea all cost fluid.
• Low blood pressure. It can feel like tiredness or light-headedness, and it was more common on retatrutide ^[3].
• Low blood sugar. This mainly applies to people with diabetes who also take insulin or sulfonylureas.

If tiredness is new and lasting, these are worth checking with a doctor rather than putting down to the drug.

Reta and alcohol

No trial has studied retatrutide together with alcohol, so there is no retatrutide-specific answer. What is known from the drug class:

• Alcohol adds to the stomach effects. Drinking on top of slowed gastric emptying can make nausea and vomiting worse.
• Pancreatitis risk. Heavy drinking is a major cause of pancreatitis, which is a listed concern for the whole GLP-1 class.
• Low blood sugar. Alcohol lowers blood sugar, which matters most for people with diabetes on other glucose-lowering drugs.
• People may drink less. A small randomized trial found that low-dose semaglutide reduced alcohol craving and how much people drank in a lab setting ^[7]. Whether retatrutide does the same hasn't been tested.

More detail is in semaglutide and alcohol, and most of it carries over to the class.

Retatrutide side effects in women

Women made up 48% of the TRIUMPH-2 trial ^[3], and the abstracts don't break side effects down by sex. Three points from the drug class are worth knowing:

• Birth-control pills. Retatrutide slows gastric emptying ^[5], which can affect how well oral medicines are absorbed. For tirzepatide, a related Lilly drug, the label advises adding a non-oral method of contraception for 4 weeks after starting and after each dose increase. Retatrutide has no label yet, so ask a doctor or pharmacist.
• Pregnancy. Pregnant women weren't enrolled in the trials, and there is no safety data for pregnancy.
• PCOS and fertility. Weight loss can restore ovulation in some women with PCOS, so pregnancy can happen sooner than expected.

How long does reta take to kick in?

There are two different answers, depending on what "kick in" means:

• Appetite. The effect on fullness comes from the first doses, since the drug slows gastric emptying ^[5]. Its half-life is about 6 days ^[6], so blood levels keep building over the first few weeks on any one dose.
• Weight. In the phase 2 obesity trial, people on 12 mg had lost 17.5% of their body weight at 24 weeks and 24.2% at 48 weeks ^[1]. Weight was still falling at 48 weeks. In TRIUMPH-1, the 80-week results were −17.6%, −23.7% and −25.0% at 4, 9 and 12 mg, against −3.9% on placebo ^[2].

The trials raised the dose in steps, so much of the first few months is spent below the final dose. Slower early weight loss is expected, not a sign the drug isn't working.

Where is retatrutide injected?

Retatrutide is a once-weekly subcutaneous injection: into the fat just under the skin, not into muscle. In TRIUMPH-2, participants injected themselves ^[3]. The usual sites for weekly subcutaneous weight-loss drugs are:

• the abdomen, at least 5 cm (2 in) from the navel,
• the front of the thigh,
• the back of the upper arm, usually done by someone else.

Rotating between sites each week reduces lumps and irritation. Our guides on subcutaneous injection and injection site rotation cover technique.

Is reta bad for you?

In the trials, retatrutide's side effects were broadly in line with other GLP-1 drugs, plus the dysesthesia and hypotension signals ^[3]. Deaths were rare and were all judged by investigators to be unrelated to the drug ^[3]. What isn't known yet is the long-term picture beyond 80 weeks, and the result of the cardiovascular outcomes trial.

The bigger risk is not the molecule but where it comes from. The trial results apply to Lilly's drug, given at known doses with medical monitoring. Retatrutide bought online is unregulated: the vial may hold the wrong amount, a different compound, or contaminants, and there is nobody to report problems to ^[8]. Until it is approved, the only legal way to take retatrutide is in a clinical trial.

For the full picture of how the drug works and where approval stands, see the retatrutide guide and the retatrutide reference page.