Semaglutide vs Tirzepatide: What the Clinical Evidence Actually Shows About These Two Weight Loss Medicines

Reviewed by GPhC Pharmacist at Priory Pharmacy, York

Two names come up again and again when patients ask us about weight loss medication: semaglutide and tirzepatide. Both are injectable medicines. Both belong to a drug class that’s transformed obesity treatment over the last five years.

But they’re not the same drug. They don’t work in exactly the same way. And the evidence doesn’t suggest you can simply swap one for the other.

Choosing between them or deciding neither is right for you should come from a proper clinical assessment, not a product comparison chart. This guide explains what the evidence actually shows, from a pharmacist’s point of view.

How They Work Differently

Semaglutide: Targets One Hormone Pathway

Semaglutide copies a natural hormone called GLP-1, which your gut releases after eating. It attaches to GLP-1 receptors throughout your body in your pancreas, brain, gut, heart, and kidneys.

What this does:

• Helps your body release insulin when blood sugar rises
• Reduces a hormone called glucagon
• Slows down digestion
• Acts on your brain’s hunger centre to reduce appetite and increase fullness

Dosing: A once-weekly injection. It starts at a low dose (0.25mg) and increases slowly over 16–20 weeks, up to the full dose (2.4mg). This slow increase helps your body adjust and reduces nausea.

Tirzepatide: Targets Two Hormone Pathways

Tirzepatide does everything semaglutide does but it also activates a second hormone receptor called GIP.

• GIP is another natural gut hormone.
• It plays a role in insulin release and how your body stores and uses fat.

Why this matters: Activating both receptors together seems to produce a bigger effect than either one alone. Scientists are still working out exactly why — but it likely involves stronger fullness signals in the brain, better pancreas function, and helpful changes in how fat tissue behaves.

Dosing: Also a once-weekly injection, starting at 2.5mg and increasing over about 20 weeks, up to a maximum of 15mg.

What the Major Trials Found

Semaglutide The STEP Trials

The STEP 1 trial tested semaglutide (2.4mg) in adults with obesity, without diabetes, over 68 weeks.

Results:

• Average weight loss: 14.9%, compared to 2.4% with placebo
• 69% of participants lost at least 10% of their body weight
• 50% lost at least 15%

In people who also had type 2 diabetes (STEP 2 trial), average weight loss was lower — around 9.6%. This is a known pattern: diabetes itself tends to reduce the weight-loss response to this drug class.

Tirzepatide The SURMOUNT Trials

The SURMOUNT-1 trial tested tirzepatide in adults with obesity, without diabetes, also over roughly 68–72 weeks.

Results (at the 15mg dose):

• Average weight loss: 20.9%, compared to 3.1% with placebo
• 91% of participants lost at least 5% of their body weight
• 57% lost at least 20%
• 36% lost at least 25%

These results were higher than anything previously achieved with a non-surgical obesity treatment — and sparked real discussion in the medical community about whether tirzepatide marks a genuine step forward.

So Which One Produces More Weight Loss?

No large trial has yet directly compared the two head-to-head as its main goal, in people without diabetes. (A trial called SURMOUNT-5 is running now and should give us that answer.)

For now, indirect comparisons suggest tirzepatide, at its higher doses, produces more average weight loss than semaglutide at its approved dose.

These are average results from specific trial groups. Individual results vary a lot. Direct head-to-head trial data isn’t published yet.

Side Effects: How Do They Compare?

Both drugs cause similar digestive side effects this comes from how they act on the gut.

Most common side effects (both drugs):

• Nausea
• Vomiting
• Diarrhoea
• Constipation

These are most common while your dose is being increased, and usually settle over time.

Reported rates in trials:

• Tirzepatide (15mg): nausea in about 31%, vomiting in about 20%
• Semaglutide: nausea in about 44%, vomiting in about 24%

(These numbers come from different trials with different setups, so treat the comparison as a rough guide, not a precise head-to-head figure.)

Rare but serious warnings for both drugs:

• A rare thyroid cancer called medullary thyroid carcinoma
• Pancreatitis
• Not safe in pregnancy
• Not suitable if you or a close family member has a history of medullary thyroid carcinoma or a condition called MEN2

Which One Is Right for You?

Honestly it depends. This should be decided through a real clinical consultation, not by comparing trial headlines.

What actually matters for your decision:

• Whether you’re clinically eligible for either drug
• Other health conditions you have
• Other medicines you’re taking
• Cost
• How well you’re likely to tolerate each option

For some patients, tirzepatide’s extra weight-loss potential will make a real difference. For others, semaglutide will work just as well and suit their situation better.

At Priory Pharmacy in York, our weight management consultations start with a full individual assessment before we recommend anything. The real question isn’t “which drug loses more weight on average” it’s “which treatment, if any, is right for you.”

Neither medicine works alone. Both require a proper consultation, ongoing monitoring, and real commitment to diet and lifestyle changes alongside treatment. These medicines work best — and most safely — as part of a structured, supervised programme.

A Note on Weight, Metabolic Health, and Sexual Function

Obesity and related metabolic conditions are well-established risk factors for erectile dysfunction, largely because both share an underlying vascular mechanism the same endothelial dysfunction that raises cardiovascular risk also restricts blood flow required for erectile function. Men starting semaglutide or tirzepatide for weight management sometimes report improvements in erectile function as weight and metabolic markers improve, though this isn’t a guaranteed outcome and shouldn’t be the primary reason for choosing either treatment.

If erectile dysfunction is a separate, ongoing concern, it’s worth addressing directly rather than waiting on weight loss alone. Priory Pharmacy offers a confidential erectile dysfunction consultation in York, with pharmacist prescribers able to assess and treat where appropriate without needing a GP referral.

This article is for general information only. It’s not personal medical advice. To discuss weight management options at Priory Pharmacy in York, please book a consultation with our clinical team.