Weight loss medications are generally safe when prescribed appropriately and monitored by a physician, but every class of these drugs carries distinct risks that vary by your health history. Some older medications were pulled from the market for causing serious heart damage, while newer options like GLP-1 drugs have shown strong safety profiles in large trials, with one study even demonstrating a 20% reduction in major cardiac events. The answer depends heavily on which medication you’re taking, what other conditions you have, and how closely you’re being monitored.
A History of Safety Failures
Weight loss drugs have a troubled track record, which is part of why people are cautious. The most infamous example is fen-phen, a combination of fenfluramine and phentermine popular in the 1990s. Fenfluramine was found to cause heart valve disease and a dangerous condition called pulmonary hypertension by activating serotonin receptors on heart cells. Fenfluramine, its chemical cousin dexfenfluramine, and the fen-phen combination were all pulled from the market in 1997. Another drug, sibutramine, was later removed for similar cardiovascular concerns. All three medications that caused the most serious heart problems worked primarily by affecting serotonin signaling in the body.
These high-profile failures led to much stricter safety requirements for modern weight loss drugs. Today’s approved medications have gone through larger and longer clinical trials before reaching the market, and post-approval monitoring is more rigorous than it was in the 1990s.
GLP-1 Medications: Benefits and Risks
The GLP-1 class, which includes semaglutide (Wegovy) and tirzepatide (Zepbound), is the most widely discussed group of weight loss drugs right now. On the safety front, these medications have performed better than many expected. A major cardiovascular trial found that semaglutide reduced heart attacks, strokes, and cardiovascular deaths by 20% over roughly 33 months in people with obesity and existing heart disease. That’s a significant protective effect, not just the absence of harm.
The most common side effects are gastrointestinal: nausea, vomiting, diarrhea, and constipation. These tend to be worst during the dose-escalation phase and often improve over time. A more serious but rarer concern is gastroparesis, a condition where the stomach empties too slowly. A study published in JAMA found that semaglutide users developed gastroparesis at a rate of about 9 per 1,000 person-years, roughly three times the rate seen with other weight loss medications. That’s still uncommon, but it’s a meaningful risk, especially for people who already have slow digestion.
GLP-1 medications also carry an FDA boxed warning, the most serious type, regarding thyroid tumors. In animal studies, these drugs caused a type of thyroid cancer called medullary thyroid carcinoma. Whether this translates to humans isn’t fully established, but the medications are not prescribed to anyone with a personal or family history of that specific cancer or a related genetic condition called MEN 2. Pancreatitis is another recognized risk. Cases of acute pancreatitis, including severe and fatal cases, have been reported in people taking these drugs.
Drug Interactions Worth Knowing About
Semaglutide alone has 376 known drug interactions, 16 of which are classified as major. Because GLP-1 drugs slow digestion, they can change how quickly your body absorbs other oral medications. This matters most for drugs where timing is critical, like thyroid hormone, blood thinners, or oral contraceptives. If you take any prescription medication regularly, your prescriber needs to review the full list before starting you on a GLP-1 drug.
People with a history of diabetic eye disease should also be aware that GLP-1 medications can worsen retinopathy, particularly during rapid blood sugar changes early in treatment.
Psychiatric Risks With Naltrexone-Bupropion
Naltrexone-bupropion (Contrave) works differently from GLP-1 drugs, targeting brain pathways involved in cravings and reward. Because it contains bupropion, the same active ingredient found in certain antidepressants and smoking cessation aids, it carries a boxed warning about suicidal thoughts. In clinical studies, a small number of people under 24 experienced increased suicidal thinking while taking bupropion.
The warning extends beyond young adults. Mental health changes, including new or worsening depression, severe anxiety, panic attacks, agitation, aggressive behavior, insomnia, and hallucinations, can occur at any age, particularly when the dose changes. Anyone with a personal or family history of bipolar disorder, schizophrenia, or prior suicide attempts needs to discuss these risks carefully before starting this medication. Close monitoring by family members or a caregiver during the first weeks of treatment is important.
Orlistat and Nutritional Gaps
Orlistat (Alli, Xenical) works by blocking fat absorption in the gut, which means about a third of the fat you eat passes through undigested. The most obvious side effects are digestive: oily stools, gas, and urgent bowel movements, especially after high-fat meals. But the less visible risk is nutritional. Because fat-soluble vitamins (A, D, E, and K) need dietary fat to be absorbed, orlistat can create deficiencies over time. The Mayo Clinic recommends taking a daily multivitamin containing all four of these vitamins plus beta carotene, timed at least two hours before or after taking the medication so the supplement isn’t affected.
What Monitoring Looks Like
Weight loss medications aren’t a “take it and forget it” situation. The Endocrine Society recommends check-ins at least monthly for the first three months, then every three months after that. These visits typically track your weight, waist circumference, and side effects. Depending on the medication, your doctor may also order blood work to check kidney function, blood sugar, liver enzymes, or thyroid markers.
This monitoring schedule exists because many side effects develop gradually. Gastroparesis can build over weeks. Vitamin deficiencies take months to become apparent. Psychiatric changes can emerge whenever a dose is adjusted. Regular follow-up catches problems before they become serious.
Safety in Adolescents
The FDA approved semaglutide and liraglutide for adolescents aged 12 and older in 2020 and 2022, respectively. Prescriptions for teens with obesity have risen substantially since then. However, long-term safety data in this age group is limited. The CDC has emphasized that postmarketing monitoring is essential to track potential unanticipated side effects as more adolescents use these medications. The developing body handles drugs differently than an adult body, and effects on growth, bone density, and hormonal development over years of use simply aren’t known yet.
Who Should Avoid Weight Loss Medications
Certain health conditions make specific weight loss drugs unsafe. GLP-1 medications are off-limits if you have a personal or family history of medullary thyroid cancer or MEN 2. Naltrexone-bupropion should not be used by people with seizure disorders, eating disorders (specifically bulimia or anorexia), or those going through abrupt withdrawal from alcohol or sedatives. Orlistat is not appropriate for people with chronic malabsorption syndromes or gallbladder disease. Phentermine, an older stimulant-type medication, is generally avoided in people with uncontrolled high blood pressure, heart disease, or hyperthyroidism.
Pregnancy is a contraindication for all weight loss medications. If you’re planning to become pregnant, most guidelines recommend stopping these drugs at least two months beforehand, though the specific timeline varies by medication.