Weight loss injections, particularly GLP-1 receptor agonists such as semaglutide (Wegovy) and liraglutide (Saxenda), are increasingly prescribed for obesity management in the UK. Whilst obesity itself significantly impairs fertility in both women and men, questions arise about whether these medications affect reproductive health. The relationship is complex: weight loss achieved through these injections may improve fertility by addressing obesity-related hormonal disruption, yet current guidance advises discontinuation before attempting conception due to limited pregnancy safety data. Understanding this balance is essential for individuals planning pregnancy whilst managing their weight.

Understanding Weight Loss Injections and Their Mechanism

Weight loss injections, primarily glucagon-like peptide-1 (GLP-1) receptor agonists, have become increasingly prescribed for obesity management in the UK. The most commonly used medications include semaglutide (marketed as Wegovy for weight loss), liraglutide (Saxenda), and tirzepatide (Mounjaro).

These injections work by mimicking the action of GLP-1, a naturally occurring hormone produced in the intestine. The mechanism involves several pathways: they slow gastric emptying, which prolongs the feeling of fullness after eating; they act on appetite centres in the brain to reduce hunger signals; and they help regulate blood glucose levels by enhancing insulin secretion when blood sugar rises.

The medications are administered via subcutaneous injection, with different dosing schedules: once weekly for semaglutide and tirzepatide, or once daily for liraglutide. Clinical trials show varying degrees of weight loss: semaglutide typically achieves approximately 15% weight reduction over 68 weeks, while liraglutide produces about 5-8% weight loss over 56 weeks.

Common side effects include nausea, vomiting, diarrhoea, constipation, and abdominal discomfort, particularly during the initial titration phase. More serious but rare adverse effects can include pancreatitis, gallbladder problems (particularly with rapid weight loss), thyroid-related issues, and in people with type 2 diabetes using semaglutide, potential worsening of diabetic retinopathy. NICE guidelines recommend these medications as part of a comprehensive weight management programme that includes dietary modification, increased physical activity, and behavioural support.

The Link Between Body Weight and Fertility

Body weight plays a crucial role in reproductive health for both women and men, with obesity recognised as a significant factor affecting fertility outcomes. In women, excess body weight can disrupt the delicate hormonal balance required for regular ovulation. Adipose tissue (fat cells) produces oestrogen, and excessive fat stores can lead to elevated oestrogen levels, which interfere with the hypothalamic-pituitary-ovarian axis—the hormonal system controlling the menstrual cycle.

Women with obesity are more likely to experience anovulation (absence of ovulation), irregular menstrual cycles, and polycystic ovary syndrome (PCOS), a common endocrine disorder affecting fertility. Research indicates that obesity reduces the success rates of both natural conception and assisted reproductive technologies such as IVF. Additionally, maternal obesity increases risks during pregnancy, including gestational diabetes, pre-eclampsia, and complications during delivery.

For men, obesity similarly affects reproductive function. Excess weight can lead to reduced testosterone levels, decreased sperm quality (including lower sperm count, reduced motility, and abnormal morphology), and erectile dysfunction. The conversion of testosterone to oestrogen in adipose tissue disrupts the hormonal environment necessary for optimal sperm production.

Conversely, achieving a healthier weight through modest weight loss (typically 5-10% of body weight) has been shown to improve fertility outcomes significantly. Studies demonstrate that weight reduction can restore regular ovulation in women, improve menstrual regularity, enhance sperm parameters in men, and increase the likelihood of successful conception. This forms the basis for NICE recommendations that weight management should be a first-line intervention for individuals with obesity who are experiencing fertility difficulties.

How Weight Loss Injections May Impact Fertility

The relationship between weight loss injections and fertility is complex and multifaceted. On one hand, the weight loss achieved through these medications may improve fertility by addressing obesity-related reproductive dysfunction. The metabolic improvements—including better insulin sensitivity, reduced inflammation, and hormonal rebalancing—that accompany weight loss could theoretically enhance reproductive function in both women and men.

However, current product information and regulatory guidance advise against using these medications during pregnancy. The manufacturer of semaglutide recommends discontinuing the medication at least two months before attempting conception, while liraglutide's guidance indicates it should be discontinued before pregnancy without specifying a fixed timeframe. This precautionary approach exists because animal studies have shown potential risks to foetal development, although human pregnancy data remains limited. The MHRA and EMA classifications indicate these medications should not be used during pregnancy.

During the active weight loss phase, particularly with rapid weight reduction, hormonal fluctuations may temporarily affect menstrual regularity and ovulation patterns. Some observational reports suggest changes in menstrual cycles whilst taking GLP-1 receptor agonists, though there is limited evidence establishing these medications as direct causes of cycle disruption beyond the effects of weight loss itself.

It is important to note that reliable contraception is essential for women of childbearing potential who are taking weight loss injections. For semaglutide and liraglutide, the UK product information does not indicate a clinically relevant effect on combined oral contraceptive effectiveness. However, tirzepatide may reduce oral contraceptive exposure, requiring additional contraceptive precautions for 4 weeks after initiation and after each dose increase. Women should discuss contraceptive options with their healthcare provider to ensure adequate pregnancy prevention during treatment.

Clinical Evidence and Research Findings

The evidence base regarding weight loss injections and fertility is still developing, with most research focusing on metabolic and cardiovascular outcomes rather than reproductive health specifically. Studies examining GLP-1 receptor agonists in women with PCOS have shown promising results: weight loss achieved with these medications has been associated with improved menstrual regularity, enhanced ovulation rates, and better metabolic parameters including insulin resistance—all factors that support fertility.

Systematic reviews of studies involving women with PCOS treated with liraglutide have demonstrated improvements in ovulation frequency and menstrual cycle regularity compared to placebo or lifestyle intervention alone. However, these studies primarily measured surrogate markers of fertility rather than pregnancy rates or live birth outcomes. The weight loss itself, rather than a direct pharmacological effect on reproductive tissues, appears to be the primary mechanism driving these improvements.

Regarding pregnancy outcomes, data remains limited. The UK product information emphasises that these medications should be avoided during pregnancy due to insufficient safety data. Animal studies showing potential foetal risks involved doses higher than those used clinically, and the relevance to human pregnancy remains uncertain. This uncertainty underlies the precautionary recommendations to discontinue these medications before conception.

For male fertility, research is even more sparse. Theoretical benefits might include improved testosterone levels and sperm parameters following weight loss, but no large-scale studies have specifically examined the effects of GLP-1 receptor agonists on male reproductive function. The general principle that weight loss improves male fertility suggests potential benefits, but direct evidence is lacking.

Clinicians and patients should recognise that current evidence does not support using these medications specifically to enhance fertility. Rather, they may be considered as part of a comprehensive weight management strategy for individuals with obesity who are experiencing fertility difficulties, with appropriate discontinuation before attempting conception.

Recommendations for Those Planning Pregnancy

For individuals considering pregnancy whilst taking or contemplating weight loss injections, careful planning and medical guidance are essential. For semaglutide, the recommendation is to discontinue treatment at least two months before attempting to conceive. For liraglutide, the product information advises discontinuation before pregnancy without specifying a fixed timeframe. This washout period allows the medication to clear from the body and provides time for metabolic and hormonal stabilisation.

Before stopping the medication, patients should discuss their fertility plans with their prescribing clinician. A structured approach might include: achieving a stable, healthier weight before discontinuation; establishing sustainable lifestyle modifications (diet and physical activity) to maintain weight loss without medication; optimising other health parameters such as blood pressure and glucose control; and commencing preconception care including folic acid supplementation (400 micrograms daily available over-the-counter, or 5 milligrams by prescription if BMI remains above 30) and vitamin D (10 micrograms daily).

For women with obesity-related fertility issues, particularly those with PCOS, weight loss injections might be considered as a bridge therapy to achieve sufficient weight reduction to improve fertility, followed by discontinuation and a period of weight maintenance before conception attempts. This approach should be undertaken with input from both weight management specialists and fertility or reproductive health clinicians. NICE guidance on fertility problems recommends that women with a BMI over 30 should be informed that weight loss may improve their chances of conception.

For men planning to father a child, there is no evidence-based requirement to discontinue weight loss injections before conception. The focus should be on achieving and maintaining a healthier weight and optimising overall health, which may benefit sperm quality.

Contraception must be reliable during treatment with weight loss injections. For semaglutide and liraglutide, combined oral contraceptives remain effective. However, tirzepatide requires additional contraceptive precautions for 4 weeks after initiation and after each dose increase. Long-acting reversible contraception (LARC) methods such as intrauterine devices or implants are unaffected by these medications and may be preferable options during treatment.

When to Seek Medical Advice

Patients should contact their GP or prescribing clinician if they are taking weight loss injections and discover they are pregnant. Whilst there is no need for alarm, the medication should be stopped immediately, and appropriate antenatal care should be arranged. Standard NHS antenatal care will be provided, including a booking appointment and dating scan at 10-14 weeks, with additional monitoring arranged if clinically indicated.

Individuals planning pregnancy should schedule a preconception consultation with their GP well in advance of stopping weight loss medication. This appointment allows for comprehensive discussion of: the optimal timing for discontinuation; strategies for maintaining weight loss; screening for other factors affecting fertility; optimisation of any co-existing conditions such as diabetes or hypertension; and initiation of appropriate preconception supplements and vaccinations.

Seek urgent medical advice if you experience severe abdominal pain, persistent vomiting, or signs of pancreatitis whilst taking weight loss injections, as these require immediate assessment regardless of pregnancy plans. Similarly, any concerning symptoms during early pregnancy following recent use of these medications should prompt medical review.

For those experiencing fertility difficulties, referral to specialist fertility services may be appropriate. NICE recommends that couples should be offered investigation if they have not conceived after one year of regular unprotected intercourse (or sooner if the woman is over 36 or there are known factors affecting fertility). Weight management, including the potential role of weight loss injections as part of a comprehensive programme, should be discussed with fertility specialists who can provide individualised advice.

Patients should maintain open communication with their healthcare team throughout their weight loss journey, particularly if fertility is a consideration. Regular monitoring appointments provide opportunities to discuss progress, adjust treatment plans, and ensure that weight management strategies align with reproductive health goals.

It's important to note that GLP-1 receptor agonists are generally not recommended during breastfeeding due to limited safety data. If you experience any suspected side effects from weight loss injections, report them to the MHRA Yellow Card scheme (yellowcard.mhra.gov.uk).

Frequently Asked Questions