Can Weight Loss Injections Cause Infertility?

Step 1: Understand what 'not licensed in pregnancy' actually means

The most important thing to be clear on: Mounjaro and Wegovy carry a clear recommendation against use in pregnancy, breastfeeding, or when actively trying to conceive. That guidance exists because there is not enough human safety data in these situations — not because trials have shown harm to fertility itself. Regulatory caution in the absence of evidence is standard for newer medicines, and it is the right call.

Animal studies did show some reproductive effects at high doses, which is why the MHRA and manufacturers take a conservative position. But animal data at elevated doses does not translate directly to human risk at licensed doses, and no large human study has recorded lasting fertility damage from either tirzepatide or semaglutide. The NHS medicine page for tirzepatide states plainly that Mounjaro is not recommended in pregnancy or when breastfeeding and advises effective contraception throughout treatment.

The practical implication is this: if there is any possibility of becoming pregnant while on treatment, reliable contraception is essential, and a conversation with your prescriber about the right method for you should happen before you start. If you are unsure how to obtain weight loss injections through a safe and clinically supervised route, our dedicated guide walks you through the process from first consultation to prescription.

Step 2: Know the contraception interaction, especially with the pill

This is the area where the science is most specific and most actionable. NHS guidance confirms that women taking tirzepatide (Mounjaro) who rely on oral contraceptive pills should add a barrier method, such as condoms, for the first four weeks of treatment and for four weeks after each dose increase. The reason is mechanical: tirzepatide slows gastric emptying, which may reduce how reliably an oral pill is absorbed. This is not an interaction that affects fertility directly; it is a temporary window during which the pill's effectiveness may be reduced, raising the risk of an unintended pregnancy.

The same pharmacological concern has not been confirmed for semaglutide (Wegovy), but NHS England's guidance on weight-management injections advises anyone on these medicines to discuss contraception and reproductive plans with a clinician before and during treatment. A non-oral method (an IUD, implant, patch, or ring) sidesteps the absorption question entirely and is worth discussing with your GP.

One thing our prescribers hear regularly: people assume the injection itself acts as a contraceptive. It does not, and anyone asking whether weight loss injections can affect fertility will find our detailed guide addresses exactly that assumption alongside the wider evidence.

Step 3: Weigh up what weight loss itself does to reproductive health

GLP-1 medicines produce significant, sustained weight loss in many people, and weight loss, achieved through any route, has complex effects on reproductive hormones. For people with obesity-related conditions such as polycystic ovary syndrome (PCOS), reducing body weight can restore more regular menstrual cycles, improve ovulation, and in some cases increase the likelihood of natural conception. That is a potential benefit, not a risk, and it is part of why effective contraception during treatment matters so much: fertility can improve before treatment ends.

You can read more about the broader picture in our guide to weight loss injections and fertility. Rapid weight loss also carries the theoretical possibility of nutritional change that could affect hormone balance, which is one more reason a dietitian or specialist's input alongside treatment is valuable. Some people also ask whether B12 injections can cause weight loss, and our guide explains how they compare to GLP-1 treatments and what role nutritional support plays.

The direction of effect, in short, is more likely to be fertility-preserving or fertility-improving in people with obesity than fertility-damaging. But the evidence base is still building, and individual circumstances vary considerably. Anyone with a known fertility condition, a history of miscarriage, or plans to conceive in the near term should discuss timing and treatment options with a reproductive health specialist as well as their prescriber.

Step 4: Plan your next steps with the right people

If you are currently on a GLP-1 medicine and thinking about trying for a baby, the recommended approach is to stop treatment before attempting conception. The wash-out period your prescriber advises will depend on which medicine you are taking and your clinical picture. Do not stop treatment without that conversation first, your weight, your health, and your family plans all feed into the right decision for you specifically.

If you are considering starting treatment and are concerned about fertility, bring that to your prescriber's attention at the outset. At nume, a real clinician reads every consultation the same day, and questions about reproductive health are exactly what that review is there to address. Our clinical team can advise on whether treatment is appropriate given your situation, which contraception approach suits your circumstances, and when it may be right to pause treatment ahead of trying to conceive.

There is also a broader group of side effects worth understanding before you start. Hair thinning is one question that often comes up alongside fertility queries, our page on whether weight loss injections cause hair loss covers what the evidence says. For anyone wanting to explore the full safety picture, including those who have seen headlines and are wondering whether weight loss injections can cause cancer, our FAQs gather the questions most commonly raised. And if you have concerns about a specific symptom during treatment, the MHRA's Yellow Card scheme is the right place to report it.

Ready to talk through your situation with a prescriber? Start your free consultation and get a same-day clinical review from a GPhC-registered Independent Prescriber.