In short: A washout period is a planned gap between your last dose of one weight-loss medicine and your first dose of another. In the UK there is no single fixed rule: depending on your circumstances, your prescriber may start the new medicine on the day your next dose would have been due, or recommend a short gap of around one to two weeks. Because the medicines have no validated dose equivalence, the new one is usually started at its lowest dose and increased gradually.

This guide, written by a Ready Health prescribing pharmacist, explains how switching between weekly GLP-1 based medicines such as tirzepatide and semaglutide is usually planned, and why the decision should always be made with your clinician.

What is a washout period?

A washout period is the time allowed for one medicine to fall to low levels in the body before another medicine with a similar action is started. Tirzepatide (brand name Mounjaro) and semaglutide (brand name Wegovy) are both long-acting, once-weekly injections, so they stay in the body for several weeks after the last dose. A planned switch aims to avoid two similar medicines building up together, which could make side effects such as nausea, vomiting and diarrhoea more likely.

How long should you wait between weight-loss medicines?

There is no national protocol that sets one washout length for everyone. UK pharmacy guidance published in 2026 describes two broadly accepted approaches when moving from tirzepatide to semaglutide:

Switching on the next scheduled dose day - the new medicine is started on the day the next dose of the old one would have been due (usually seven days after the last weekly dose). This may suit people who are stable and tolerating treatment well.
A gap of around one to two weeks - often preferred if you have had troublesome side effects, so your body has time to settle first.

Your prescriber may suggest a different plan if there is a clinical reason, for example if you have recently been unwell, have kidney problems, or have stopped treatment for some time already.

What dose will I start on after switching?

Tirzepatide acts on two hormone receptors (GLP-1 and GIP), whereas semaglutide acts on one (GLP-1). Because they work differently, there is no reliable way to convert a dose of one into a dose of the other. In practice, most people restart the new medicine at its starting dose and then increase it step by step, usually every four weeks, as set out in the product licence.

This can feel like going backwards, but starting low helps your stomach adjust and reduces the chance of side effects. Your weight may plateau or rise slightly during this time, which is something to discuss openly with your prescriber.

Is switching from an injection to a tablet any different?

Daily tablets for weight management are now licensed in the UK, including an oral form of semaglutide and a newer medicine called orforglipron. When the MHRA authorised oral semaglutide for weight management in 2026, it noted that some people using the 2.4 mg weekly semaglutide injection could move directly to the highest-strength daily tablet because it is the same active ingredient. Switching between different active ingredients, for example from tirzepatide injections to a tablet, may need a washout and a lower starting dose. Tablets also have specific instructions on how and when to take them, so your prescriber will go through these with you.

What safety checks matter when you switch?

Contraception: the MHRA advises that anyone who could become pregnant uses effective contraception while taking these medicines. Tirzepatide may make oral contraceptive pills less effective in people living with overweight or obesity, so a non-oral or barrier method is advised for four weeks after starting tirzepatide and for four weeks after each dose increase.
Planning a pregnancy: the MHRA advises stopping semaglutide at least two months, and tirzepatide at least one month, before trying to conceive. These medicines should not be used during pregnancy or breastfeeding.
Pancreatitis: in January 2026 the MHRA strengthened warnings about acute pancreatitis with all GLP-1 and GLP-1/GIP medicines. Seek urgent medical help for severe, persistent tummy pain, especially if it spreads to your back or comes with vomiting.
Surgery or sedation: tell the team if you are having a procedure, as these medicines slow stomach emptying.
Other medicines: if you take insulin or a sulfonylurea for diabetes, your doses may need reviewing to reduce the risk of low blood sugar.

Why plan your switch with a clinician?

A prescriber will review your medical history, your current dose, how well you have tolerated treatment and your reasons for switching. They can then agree the timing, the new starting dose and a follow-up plan. Switching should never involve using two GLP-1 based medicines at the same time, and medicines should only ever be obtained on a valid prescription from a registered UK pharmacy or clinic.

Frequently asked questions

Do I always have to wait two weeks before switching?

No. Many people switch on the day their next dose would have been due, while others are advised to leave a gap of one to two weeks. Your prescriber will recommend the option that suits you.

Can I start the new medicine at the same strength I was on before?

Usually not. There is no validated dose equivalence between tirzepatide and semaglutide, so the new medicine is normally started at its lowest dose and increased gradually.

What if I have missed several doses before switching?

Tell your prescriber. A longer break means your body may have lost its tolerance, so restarting at a low dose is especially important.

Will switching affect my contraception?

It can. If you are moving to tirzepatide, the MHRA advises using a non-oral or barrier method for four weeks after starting and after each dose increase.

Key takeaways

A washout period is a planned gap between stopping one weight-loss medicine and starting another.
Common approaches are switching on the next scheduled dose day, or waiting around one to two weeks.
The new medicine is usually started at its lowest dose because there is no reliable dose conversion.
Review contraception, pregnancy plans and other medicines whenever you switch.
A prescriber should always decide whether a switch is suitable and plan the timing with you.

Sources and further reading

Last reviewed: October 2026 by Shamir Patel, Independent Prescribing Pharmacist, Ready Health. This article is general information and does not replace advice from your own clinician. Brand names are trademarks of their respective owners.

If you are thinking about changing your weight-management treatment, you can book a weight-management consultation at Ready Health in Standish, near Wigan. A prescriber will review your health and history and decide with you whether any treatment, or a change in treatment, is suitable.

Washout Periods When Switching Weight-Loss Medication: What to Expect