Do weight loss injections affect birth control? This is an important question for anyone using or considering GLP-1 medications such as Wegovy, Saxenda, or tirzepatide whilst relying on contraception. Weight loss injections work by slowing gastric emptying and reducing appetite, which has raised concerns about whether they might interfere with oral contraceptive absorption. The evidence varies by medication: semaglutide and liraglutide show no clinically relevant effects on oral contraceptive exposure, whilst tirzepatide requires additional contraceptive precautions. Understanding these interactions helps you make informed decisions about contraception during weight loss treatment. This article examines the evidence, explores contraceptive options, and explains when to seek professional advice.

How Weight Loss Injections Work in Your Body

Weight loss injections prescribed in the UK typically contain glucagon-like peptide-1 (GLP-1) receptor agonists, such as semaglutide (Wegovy) or liraglutide (Saxenda), or dual GIP/GLP-1 receptor agonists like tirzepatide. These medications mimic naturally occurring hormones that regulate appetite and blood glucose levels. When administered subcutaneously, usually once weekly or daily depending on the formulation, they work through several complementary mechanisms to support weight management.

The primary actions include:

• Slowing gastric emptying, which means food remains in your stomach longer, promoting feelings of fullness

• Acting on appetite centres in the brain to reduce hunger signals and food cravings

• Improving insulin sensitivity and glucose metabolism in people with or at risk of type 2 diabetes

• Potentially influencing eating behaviours through effects on brain signalling pathways

These medications are absorbed systemically and distributed throughout the body via the bloodstream. The delayed gastric emptying effect is particularly relevant when considering potential interactions with other medications, as it can theoretically affect how quickly oral medicines are absorbed from the gastrointestinal tract. This pharmacological property has raised questions about whether weight loss injections might interfere with the effectiveness of oral contraceptive pills.

It's important to understand that these medications do not directly interact with reproductive hormones such as oestrogen or progesterone. They do not alter the hormonal mechanisms that prevent ovulation or thicken cervical mucus. According to the product information for semaglutide and liraglutide, these specific medications do not show clinically relevant effects on combined oral contraceptive exposure. However, tirzepatide has a more pronounced effect on gastric emptying and carries specific warnings about oral contraceptive absorption. Additionally, the gastrointestinal side effects—particularly nausea and vomiting—warrant careful consideration for anyone relying on oral contraception.

What the Evidence Shows About Contraception and GLP-1 Medications

The evidence regarding interactions between weight loss injections and hormonal contraception varies by specific medication. For semaglutide (Wegovy) and liraglutide (Saxenda), clinical studies have shown no clinically relevant effect on the exposure of combined oral contraceptives containing ethinylestradiol and levonorgestrel. However, tirzepatide has been shown to reduce exposure to oral contraceptives, which is reflected in its product information.

The theoretical risk centres primarily on oral contraceptives: if gastric emptying is significantly delayed, the absorption window for oral medications could potentially be affected. This risk is most pronounced during dose escalation and following dose increases, when gastrointestinal effects are typically strongest.

Key evidence considerations include:

• Gastric emptying effects vary by medication: tirzepatide has more pronounced effects than semaglutide or liraglutide

• Gastrointestinal side effects (nausea, vomiting, diarrhoea) may independently affect oral contraceptive absorption

• Individual variation exists in how people respond to these medications

• Weight loss itself can alter hormone levels and potentially affect fertility

Product information for these medications notes that slowed gastric emptying may impact the absorption of oral medicines. The Faculty of Sexual and Reproductive Healthcare (FSRH) has issued guidance specifically addressing GLP-1/GIP agents and hormonal contraception, recommending different precautions based on the specific weight loss medication used.

While there is no established pattern of contraceptive failure with these medications in clinical practice, the precautionary approach varies by agent. For tirzepatide, additional contraceptive precautions are specifically recommended after initiation and dose increases, while routine additional precautions are not generally required for semaglutide or liraglutide based on current evidence.

Contraceptive Options to Consider While Using Weight Loss Injections

If you're using or planning to start weight loss injections, discussing your contraceptive method with a healthcare professional is advisable, particularly if pregnancy prevention is essential for you. Several contraceptive options are unaffected by gastric emptying or gastrointestinal absorption, providing reliable alternatives during treatment.

Long-acting reversible contraceptives (LARCs) are highly effective options and are completely independent of gastrointestinal absorption:

• Intrauterine devices (IUDs): Both copper IUDs and hormonal IUS (such as Mirena or Kyleena) provide highly effective contraception for 3–10 years depending on type

• Contraceptive implant: A small rod inserted under the skin of your upper arm, effective for three years

• Contraceptive injection: Depo-Provera administered every 12–13 weeks, though this may be less suitable if you're concerned about potential weight effects

These methods have failure rates below 1% with typical use and require no daily action, eliminating concerns about absorption or user error.

For those using oral contraceptives, precautions depend on which weight loss medication you're taking:

• With semaglutide (Wegovy) or liraglutide (Saxenda): No routine additional contraception is required based on product information

• With tirzepatide: Use a non-oral contraceptive method or add barrier methods (condoms) for 4 weeks after starting treatment and for 4 weeks after each dose increase

If you experience vomiting or diarrhoea while taking oral contraceptives:

• If you vomit within 3 hours of taking your pill, follow the missed pill guidance for your specific pill type

• If you have severe diarrhoea lasting more than 24 hours, follow missed pill guidance

• Consider emergency contraception if you've had unprotected intercourse during this time

Barrier methods alone (condoms, diaphragms) remain unaffected by weight loss medications but have higher typical-use failure rates (13–18%) and require consistent, correct use. The contraceptive patch and vaginal ring are absorbed through skin or vaginal mucosa respectively, bypassing the gastrointestinal tract entirely. However, be aware that the effectiveness of the patch may be reduced in women with higher body weight (typically >90kg); discuss this with your healthcare provider if relevant.

When to Speak with Your GP or Contraception Clinic

Proactive discussion with healthcare professionals is essential before starting weight loss injections, particularly if you're of reproductive age and sexually active. Your GP, practice nurse, or local sexual health clinic can provide personalised advice based on your medical history, current contraceptive method, and individual circumstances.

You should arrange a consultation if:

• You're currently using oral contraceptive pills and planning to start or have recently started weight loss injections

• You're experiencing significant nausea, vomiting, or diarrhoea that might affect medication absorption

• You're concerned about contraceptive reliability during weight loss treatment

• You've had unprotected intercourse and are uncertain about your contraceptive protection

• You're considering pregnancy in the near future, as weight loss medications should be discontinued before conception

• Your periods become irregular or you experience potential pregnancy symptoms

If planning pregnancy, follow medication-specific guidance:

• Semaglutide (Wegovy): Stop at least 2 months before planned conception

• Liraglutide (Saxenda): Discontinue before conception (check product information)

• Tirzepatide: Follow product information (typically at least 1 month before planned conception)

Seek urgent medical advice if you suspect you might be pregnant whilst using weight loss injections. Although data on pregnancy outcomes with exposure to these medications is limited, they are not recommended during pregnancy. Early antenatal care is important for optimal maternal and foetal health.

Your contraception review is an opportunity to discuss the full range of available methods and identify the most suitable option for your circumstances. Healthcare professionals can explain the evidence, address your specific concerns, and help you make an informed decision. Many people find that switching to a LARC method during weight loss treatment provides peace of mind and eliminates daily medication routines.

Remember that weight loss itself may increase fertility, particularly in women with polycystic ovary syndrome (PCOS) or obesity-related ovulatory dysfunction. If pregnancy prevention is important, ensuring robust contraception throughout your weight loss journey is crucial. Sexual health clinics offer confidential, specialist contraceptive advice and may be able to arrange fitting of IUDs or implants, though availability varies by clinic.

If you experience any suspected side effects from your weight loss medication, report them through the MHRA Yellow Card Scheme.

Frequently Asked Questions