Switching between weight loss medicines: what to know

This is one of the most common questions asked about weight-management medicines, and it has an uncomfortable answer: there is no official guidance.

Not “the guidance is complicated”. There isn’t any. The Pharmaceutical Journal states plainly that no NICE or MHRA guidance specifies how to switch between tirzepatide and semaglutide ^[1], and the Summaries of Product Characteristics for both medicines describe how to start them, not how to convert from something else ^[2][3].

This page explains what that gap means. It is information, not advice, and any switch is a decision for a prescriber.

Why there is no conversion chart

Dose-equivalence tables exist for medicines where the pharmacology makes them comparable — different opioids, different steroids. Semaglutide and tirzepatide do not sit in that relationship.

Semaglutide acts on the GLP-1 receptor. Tirzepatide acts on GLP-1 and GIP receptors. The doses are not on the same scale (15 mg of tirzepatide is not “more” than 2.4 mg of semaglutide in any meaningful sense), the trials were never designed to establish equivalence, and tolerating one at a high dose does not guarantee tolerating the other.

What clinicians generally do

In the absence of guidance, published clinical opinion converges on a cautious default: start the new medicine at its licensed starting dose and escalate normally ^[1].

That feels frustrating if you have spent four months climbing to a maintenance dose. The reasoning is that the gastrointestinal side effects of these medicines cluster at initiation and dose increases, and there is no evidence that tolerance transfers between them. Starting high risks a level of nausea and vomiting that would have been avoidable.

Be clear that this is expert opinion, not regulation. A prescriber who knows your history may reasonably do something different and should be able to explain why.

Reasons a switch might be considered

• Side effects that have not settled after a fair period at a tolerable dose
• Inadequate response — the SmPCs contain review points, such as reconsidering tirzepatide if less than 5% of body weight has been lost six months after reaching the highest tolerated dose ^[2]
• Supply problems, which have affected every medicine in this class at some point
• Cost, where one option becomes unaffordable
• A change in clinical circumstances — a new diagnosis, a new medicine, a plan to conceive

Wanting to try the newest thing is not a clinical reason, and a prescriber who agrees to it without discussion is not doing their job.

The gap in the middle

Both weekly injections stay in the body for some time after the last dose, then fade. The new medicine then starts at a low dose and builds over months.

So there is usually a period — weeks, not days — when appetite suppression is weaker than you have become used to. People are often caught out by this and read it as the new medicine “not working”. It is worth expecting:

• Hunger returning noticeably before the new medicine takes effect
• A short-term stall or small regain on the scales
• The same early side effects you had the first time, as the new medicine escalates

Planning meals and structure for that window matters more than it does at any other point in treatment.

What not to do

Do not switch yourself. Buying a different medicine online to change over without a prescriber involved removes the one person who has your history in front of them.

Do not overlap doses. Taking the old and new medicine together is not a smoother transition; it is a double dose of the same physiological effect.

Do not stockpile to bridge a gap. Using someone else’s pen, or a pen bought outside a registered pharmacy, is where counterfeit products enter the picture.

Do not assume your old dose transfers. This is the mistake with the clearest consequences.

Questions worth asking your prescriber

• What dose will I start on, and how quickly will it go up?
• How long should I leave between the last dose of one and the first of the other?
• What should I expect in the gap?
• What are we measuring to decide whether this switch worked, and by when?
• If this one does not suit me either, what happens next?

Safety information

The warnings that apply to each medicine apply through a switch: severe, persistent abdominal pain needs urgent attention because of the pancreatitis risk highlighted by the MHRA in January 2026 ^[6]; these medicines are not for use in pregnancy; and delayed gastric emptying can affect other oral medicines, including contraceptives. Suspected side effects can be reported through the Yellow Card scheme ^[7].

Where this information comes from

The absence of official guidance is documented in The Pharmaceutical Journal ^[1] and confirmed by the SmPCs themselves. Review points come from the products’ SmPCs, and the NHS access position from the NICE technology appraisals.

This article is for general information and isn’t a substitute for advice from your own clinician. If you feel unwell, contact your GP or NHS 111. In an emergency, call 999.