Aspirin does not cause weight gain. It is not listed as a side effect on FDA labeling, and the largest clinical trial studying daily aspirin use found no meaningful difference in body weight between aspirin users and those taking a placebo over nearly five years. If anything, the pharmacology of aspirin leans slightly in the opposite direction, with several mechanisms that could theoretically favor weight loss rather than gain.
What the Largest Trial Shows
The ASPREE trial randomized over 19,000 older adults to either 100 mg of daily aspirin or a placebo and followed them for a median of 4.7 years. At baseline, both groups were essentially identical: average body weight was 77 kg, average BMI was 28.1, and average waist circumference was 97 cm. Over the course of the trial, aspirin use did not produce any detectable shift in body weight compared to placebo. This is the strongest direct evidence available, and it shows no connection between daily low-dose aspirin and weight change.
Why Aspirin Might Cause Minor Fluid Retention
Aspirin belongs to the NSAID family, and all NSAIDs can cause the body to hold onto a small amount of extra sodium and water. Normally, certain signaling molecules in the kidneys help flush sodium out and limit water reabsorption. NSAIDs block the production of those molecules, which can tip the balance toward mild fluid retention.
In healthy people, this effect is self-correcting and typically amounts to gaining 0.5 to 1 kg (roughly 1 to 2 pounds) of water weight. The body adjusts hormone levels to compensate, and the retention plateaus quickly. It becomes more noticeable in people with heart failure, liver cirrhosis, or kidney problems, where the body’s ability to self-correct is already compromised. This is water weight, not fat gain, and it generally resolves when the medication is stopped.
Aspirin’s Effects on Metabolism and Fat
The biological effects of aspirin and related salicylate compounds actually point away from weight gain. Salicylate is a metabolic stimulant. Research published in Nature found that it can increase basal metabolic rate, and in hypothyroid patients, high doses were able to restore metabolic rate to normal levels. At the low doses most people take (75 to 325 mg daily), this effect is far less pronounced, but it certainly doesn’t suggest a pathway toward gaining weight.
Aspirin also improves the way the body responds to insulin. In animal models of obesity and insulin resistance, aspirin treatment improved both fasting blood sugar and post-meal blood sugar levels by reducing inflammation that interferes with insulin signaling. Better insulin sensitivity generally means the body is more efficient at using glucose for energy rather than storing it as fat.
A related compound called salsalate (a prodrug of salicylate) has been shown to activate brown fat in mice. Brown fat burns calories to generate heat rather than storing energy the way regular fat does. Salsalate increased the uptake of fatty acids into brown fat tissue, raised body temperature, and boosted the expression of a protein responsible for calorie-burning heat production. While salsalate is not identical to aspirin, both share the same active metabolite, and this research further suggests that salicylates work against fat accumulation rather than promoting it.
Appetite and Stomach Effects
Aspirin is well known for irritating the stomach lining. One of its recognized side effects, listed by the Mayo Clinic, is loss of appetite. For some people, especially those taking higher doses or using aspirin without food, the GI discomfort can reduce how much they feel like eating. This would, if anything, push caloric intake downward. There is no evidence that aspirin increases appetite or cravings.
Other Medications May Be the Culprit
People who take daily aspirin often take other medications too, particularly for heart disease, high blood pressure, or diabetes. Several of those drugs are well-established causes of weight gain. Corticosteroids like prednisone cause fluid retention and increased appetite. Beta-blockers can slow metabolism. Certain diabetes medications promote fat storage. If you’ve started aspirin around the same time as another new medication and noticed weight changes, the other drug is a far more likely explanation.
Lifestyle changes that coincide with a new aspirin prescription can also play a role. A new cardiovascular diagnosis sometimes leads to reduced physical activity or stress-related eating patterns. These are easy to attribute to “the new pill” when they actually stem from the circumstances around it.