• 29 Apr, 2026
• SendSlim Team
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Common Side Effects of Weight Loss Medications

What to Know Before You Start a Weight Loss Medication

Weight loss medications can be powerful tools when used under medical supervision. But like all medications, they come with possible side effects. Knowing what to expect before you start can help you feel more prepared and make better decisions about your health. This article covers the most common side effects of popular weight loss medications and what you can do to manage them.

It is important to remember that not everyone experiences side effects the same way. Some people have mild symptoms that go away on their own. Others may need to adjust their dose or switch medications. Working closely with a licensed clinician is the best way to find what works for your body safely.

Types of Weight Loss Medications and How They Work

There are several types of FDA-approved weight loss medications. Some of the most commonly prescribed include GLP-1 receptor agonists like semaglutide (Wegovy, Ozempic) and tirzepatide (Zepbound, Mounjaro). These medications work by mimicking hormones in the gut that tell your brain you are full. They also slow how fast your stomach empties food, which helps reduce hunger and calorie intake.

Other older medications include orlistat (Xenical, Alli), which blocks fat absorption in the gut, and phentermine, which works as an appetite suppressant by affecting the central nervous system. Bupropion-naltrexone (Contrave) is another option that affects hunger signals in the brain. Each medication works differently, which means the side effects can vary a lot depending on which one you take.

Digestive Side Effects: The Most Common Complaints

The most frequently reported side effects from weight loss medications involve the digestive system. For GLP-1 receptor agonists like semaglutide and tirzepatide, nausea is the number one complaint, especially when first starting the medication or after a dose increase. Some people also experience vomiting, diarrhea, constipation, or stomach pain. These symptoms often happen because the medication slows digestion.

For orlistat, digestive side effects can be more intense. Because the medication blocks fat from being absorbed, undigested fat passes through the bowels. This can cause oily or loose stools, frequent bowel movements, gas, and oily spotting. These effects are more likely to happen when eating high-fat meals. Following a low-fat diet while taking orlistat can greatly reduce these symptoms.

In most cases, digestive side effects from GLP-1 medications improve over time as your body adjusts. Starting at a low dose and slowly increasing it — called dose titration — is one of the most effective ways to reduce nausea and stomach discomfort. Eating smaller meals and avoiding greasy or spicy foods can also help.

Other Common Side Effects to Watch For

Beyond digestive issues, there are other side effects worth knowing about. People taking phentermine may experience increased heart rate, dry mouth, difficulty sleeping, or feelings of nervousness. These effects are related to how the medication stimulates the nervous system. Because of this, phentermine is not recommended for people with certain heart conditions.

Bupropion-naltrexone (Contrave) can cause headaches, dizziness, dry mouth, and constipation. It also carries a warning about increased risk of suicidal thoughts, which is the same warning found on other bupropion-containing products. This is why regular check-ins with a clinician are so important when taking this medication.

Some people taking GLP-1 medications report fatigue, headaches, or dizziness, particularly in the early weeks. These symptoms often pass. However, any new or worsening symptoms should always be reported to your healthcare provider right away.

Rare But Serious Side Effects

While most side effects are mild and manageable, there are some rare but serious risks to be aware of. GLP-1 receptor agonists carry a warning about a possible risk of thyroid tumors, including a rare cancer called medullary thyroid carcinoma, based on animal studies. Because of this, these medications are not recommended for people with a personal or family history of certain thyroid cancers or a condition called Multiple Endocrine Neoplasia type 2 (MEN 2).

Pancreatitis, or inflammation of the pancreas, has been reported in some people using GLP-1 medications. Symptoms include severe stomach pain that may spread to the back, nausea, and vomiting. This is rare, but it is important to seek medical care immediately if these symptoms occur. Gallbladder problems, including gallstones, have also been reported with rapid weight loss and with GLP-1 medications specifically.

Orlistat has been associated with rare cases of serious liver injury, though the link is not fully confirmed. Any yellowing of the skin or eyes, dark urine, or severe stomach pain while taking orlistat should be evaluated by a doctor promptly.

How to Manage Side Effects

Many side effects can be managed with simple strategies. For nausea from GLP-1 medications, eating bland foods like crackers or toast, avoiding large meals, and staying hydrated can make a real difference. Taking your medication at the same time each week and not eating right before bedtime may also help. Ginger tea or ginger candies are a natural remedy some people find soothing for nausea.

Staying in regular contact with your prescribing clinician is one of the most important steps you can take. If side effects are affecting your quality of life, your dose may be adjusted or a different medication may be considered. You should never stop a prescribed medication without first talking to your healthcare provider.

If you are in California or Nevada and looking for accessible medical guidance, SendSlim offers video and phone visits with licensed clinicians for just $50 per visit — no insurance required. Having a knowledgeable clinician on your side makes managing side effects much easier and safer.

When to Call Your Doctor

Some symptoms should never be ignored. Contact your doctor or seek emergency care if you experience severe stomach pain, signs of an allergic reaction (such as rash, swelling, or trouble breathing), chest pain, or rapid heartbeat. These could be signs of a serious reaction that needs immediate attention.

For less urgent concerns, it is still a good idea to keep a simple log of any symptoms you notice, including when they started, how long they last, and how severe they are. This information helps your clinician adjust your treatment plan. SendSlim makes it easy to connect with a licensed provider whenever questions or concerns come up, without the hassle of a traditional clinic visit.

Final Thoughts

Weight loss medications can be effective when used correctly and under proper medical supervision. Understanding possible side effects helps you use these medications more safely and confidently. Most side effects are manageable and improve with time, but staying connected with a healthcare provider is always the smartest approach. Knowledge is one of the most powerful tools you have on your wellness journey.

References

• Wilding, John P.H. et al. “Once-Weekly Semaglutide in Adults with Overweight or Obesity.” New England Journal of Medicine. 2021.
• U.S. Food and Drug Administration. “Medications Target Long-Term Weight Control.” FDA.gov. 2023.
• Khera, Rohan et al. “Association of Pharmacological Treatments for Obesity with Weight Loss and Adverse Events.” JAMA. 2016.
• National Institute of Diabetes and Digestive and Kidney Diseases. “Prescription Medications to Treat Overweight and Obesity.” NIH.gov. 2023.
• Jastreboff, Ania M. et al. “Tirzepatide Once Weekly for the Treatment of Obesity.” New England Journal of Medicine. 2022.