Medicine Impact on Weight: What You Need to Know

Some medicines change your weight without warning. That can be frustrating when you’re doing everything else right. This guide explains which drugs commonly add or drop pounds, why that happens, and simple actions you can take now.

Which medicines cause weight gain?

Certain drug classes are more likely to cause weight gain. Antipsychotics such as olanzapine and clozapine often increase appetite and slow metabolism. Some antidepressants — especially mirtazapine and paroxetine — can make you hungrier. Mood stabilizers (lithium, valproate), some antiepileptics (gabapentin), and older antihistamines also link to weight gain. Corticosteroids like prednisone cause rapid weight increases through fluid retention and higher appetite. For people with diabetes, insulin and sulfonylureas can lead to weight gain because they help the body store more glucose as fat.

Weight gain can come from different mechanisms: increased appetite, changes in where the body stores fat, slower resting metabolism, or fluid retention. The timing varies — steroids can cause quick gains in days to weeks, while antipsychotic-related weight gain may appear over months.

Which medicines cause weight loss?

Some drugs reduce appetite or increase calorie burn. Stimulant medications used for ADHD (amphetamine, methylphenidate) usually lower appetite. Newer diabetes drugs called GLP-1 receptor agonists (semaglutide, liraglutide) often cause steady weight loss by reducing hunger and slowing stomach emptying. Other meds that can cause weight loss include topiramate, bupropion, and sometimes metformin or SGLT2 inhibitors for diabetes.

Weight loss from medicine may be healthy or not — rapid unintended loss needs attention. If you lose more than 5% of body weight in a month or feel weak or dizzy, contact your clinician.

So what should you do if a medicine is changing your weight?

Practical steps to manage med-related weight change

1) Track weight and symptoms weekly. Quick changes are easier to address. 2) Talk to the prescriber before stopping or changing any drug — don’t quit suddenly. Ask about alternatives with lower weight effects (for example, switching antidepressants or trying different diabetes meds). 3) Ask a pharmacist about side-effect profiles and whether timing or dose changes help. 4) Use small lifestyle moves: prioritize protein at meals, add strength training twice a week to protect muscle, and keep added sugars and late-night snacking in check. 5) If fluid retention is the issue, your doctor may adjust dose or add treatments to reduce swelling.

If weight change is severe, sudden, or tied to other symptoms like breathlessness, swelling, extreme fatigue, or mood change, seek medical help fast. For gradual problems, a clear conversation with your clinician and a practical plan (med review + small habit changes) usually gets things back on track.

Need help preparing for that talk with your doctor? Make a short list of when the change started, how much weight changed, appetite or sleep shifts, and any other new meds. That list will make the visit much more productive.