Dosing & Usage

Dosing & Usage

Practical guides for using weight loss injections in the UK, including dosing schedules, missed doses, restarting after a break, and how to inject safely.

Whether you are about to start a weight-loss injection or are deep into treatment looking for a specific reference, this is the dosing and usage hub for Mounjaro and Wegovy in the UK. Each section below summarises an aspect of dosing, technique, or treatment management and links to a deeper guide. LetsLoseWeight is an independent comparison site; we do not prescribe medication, and the instructions inside your specific pen pack take precedence over anything written here.

Mounjaro dosing

Mounjaro (tirzepatide) is given as a once-weekly subcutaneous injection. UK pre-filled pens are available at six dose strengths: 2.5mg, 5mg, 7.5mg, 10mg, 12.5mg, and 15mg.

The licensed approach is to start at 2.5mg for 4 weeks, then step up by 2.5mg increments at intervals of at least 4 weeks until reaching a maintenance dose. Common maintenance doses are 5mg, 10mg and 15mg. Many patients settle on a lower maintenance dose than the maximum.

For the full schedule, why each dose level matters, and what side effects to expect at each step, see our Mounjaro dosing guide.

Wegovy dosing

Wegovy (semaglutide 2.4mg) is given as a once-weekly subcutaneous injection. UK pre-filled pens are available at five dose strengths: 0.25mg, 0.5mg, 1mg, 1.7mg, and 2.4mg.

The licensed approach is a 16-week dose escalation to reach the 2.4mg maintenance dose. If 2.4mg is not tolerated, the SmPC permits 1.7mg as a long-term maintenance dose with slightly lower efficacy.

For the full schedule and detail, see our Wegovy dosing guide.

Retatrutide dosing

Retatrutide is not licensed in the UK at the time of this guide's review. Phase 2 trial doses ranged from 1mg to 12mg weekly, with greater weight loss at higher doses. Until the MHRA approves retatrutide, no UK dosing schedule exists.

For more on retatrutide's investigational status, see our retatrutide dosing guide and retatrutide UK availability pages.

How to inject

Both Mounjaro and Wegovy are subcutaneous injections — into the layer of fat just under the skin. The licensed injection sites are the abdomen, front of the thigh, and back of the upper arm. Pen design varies; the instructions inside each pen pack include pictorial step-by-step guidance.

Key practical points:

  • Take the pen out of the fridge 15–30 minutes before injecting (a cold injection is more uncomfortable)
  • Inspect the medicine — should be clear and colourless
  • Inject perpendicular to the skin, hold for the time specified
  • Dispose of used pens in a sharps bin

For full step-by-step technique, see our how to inject guide.

Where to inject and how to rotate

Rotation between injection sites prevents lipohypertrophy (lumps under the skin), reduces local skin reactions, and ensures consistent absorption.

A simple system: rotate between abdomen, thigh, and upper arm — and rotate within each area. Avoid the same exact spot two weeks in a row.

For the full picture including a clock-face rotation system and what to avoid, see our where to inject guide.

Missed doses

The rules are different for Mounjaro and Wegovy:

  • Mounjaro: take a missed dose within 4 days (96 hours); after that, skip and resume on schedule
  • Wegovy: take a missed dose within 2 days (48 hours); after that, skip and resume on schedule

Two or more consecutive missed doses usually require dropping back a dose level. For full detail including travel and changing your injection day, see our missed dose guide.

Restarting after a break

After a gap of more than 2–4 weeks, you should not simply pick up at your previous dose. Tolerance fades, and resuming at the same dose can produce nausea and gastrointestinal upset comparable to your first weeks on the medicine. Standard advice:

  • 2–4 weeks gap: drop back one dose level and re-titrate
  • 4+ weeks gap: restart at the starter dose

For full detail, see our restarting weight loss injections guide.

Stopping safely

Mounjaro and Wegovy are not addictive; stopping does not cause withdrawal symptoms. The realistic outcome of stopping is gradual weight regain over 12–18 months — on average, around two-thirds of lost weight regained within 12 months according to the STEP 1 trial extension.

Tapering is not medically required but is sometimes used as a practical bridge. Lifestyle changes that have become embedded during treatment provide the best chance of maintaining results.

For full detail including the NHS 2-year cap on Wegovy, see our stopping safely guide.

Switching between medicines

If switching from Wegovy to Mounjaro (or vice versa), the standard approach is:

  • Stop the current medicine
  • Wait around 5 weeks (Wegovy → Mounjaro)
  • Start the new medicine at its standard starter dose

The medicines act on different combinations of receptors and doses do not equate, so the new medicine starts at its standard starter dose regardless of where you were on the previous one.

For more, see our switching from Wegovy to Mounjaro guide.

Storage and disposal

Storage

  • Unused pens: refrigerated at 2–8°C in original packaging
  • Mounjaro out of fridge: up to 21 days at room temperature (below 30°C)
  • Wegovy out of fridge: up to 28 days at room temperature (below 30°C) once first used
  • Never freeze — frozen pens must be discarded
  • Protect from light — keep pens in their original packaging until use

Disposal

  • Used pens go into a sharps bin
  • A full sharps bin can be returned to most UK community pharmacies for disposal
  • Do not recap used pens — the safety guard locks automatically

Travel

Both medicines can be transported. Practical considerations:

  • Within the UK: standard delivery and home use rules apply
  • Within the out-of-fridge window (21 days for Mounjaro, 28 days for Wegovy): no special handling required for typical trips
  • Longer trips: consider an insulated cool bag with a temperature monitor
  • Flying: pens can be carried in hand luggage with a copy of your prescription
  • Multiple time zones: pick a stable day in the new time zone — the medicine doesn't track UK time

Drug interactions to flag

Most medicines have no interaction with Mounjaro or Wegovy. Specific points worth flagging to your prescriber:

  • Oral contraceptives — Mounjaro specifically warns about reduced absorption; consider non-oral contraception or an additional barrier method for 4 weeks after starting and after each dose increase. Wegovy does not carry the same SmPC warning.
  • Insulin and sulfonylureas (in type 2 diabetes) — risk of hypoglycaemia; usually need dose reduction in other diabetes medicines
  • Other oral medicines — slowed gastric emptying may delay absorption of any oral medicine; usually not clinically significant but worth knowing

For the full eligibility and contraindication picture, see our eligibility guide.

Frequently asked questions

How long until I'm at the maintenance dose?
For Wegovy, 17 weeks (the 16-week titration plus the first week at 2.4mg). For Mounjaro, depends on the maintenance dose target — 5 weeks if you settle on 5mg, up to 21+ weeks if you target 15mg.

What's the maximum dose I can take?
For Mounjaro, 15mg per week is the licensed maximum for weight management. For Wegovy, 2.4mg is the maintenance dose and the licensed maximum. A regulated UK pharmacy will not dispense above these.

Can I split doses or take half-doses?
No. The pre-filled pens are designed to deliver a full dose only. There is no licensed way to take a partial dose.

Do I need to inject at the same time of day?
No. Same day of the week matters; time of day doesn't. Some patients prefer evening (so any nausea is overnight); others prefer morning. Pick what works.

Is the medicine still effective if I forget for one week?
A single missed dose has minimal long-term impact. Repeated missed doses (multiple per month) are a problem. The medicine works because of consistent weekly exposure.

Next steps

Use the links above to dive into any specific area, or:

Sources

This guide is for general information only and is not a substitute for prescriber advice. The instructions inside your specific pen pack take precedence over anything written here.

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Weight Loss Injection Dosing & Usage UK Hub | LetsLoseWeight