Biologics can cause modest weight gain, but the amount depends heavily on which type you’re taking and what condition is being treated. A large meta-analysis of TNF-blocking biologics found an average gain of about 1.5 kilograms (roughly 3.3 pounds) across study periods ranging from one month to two years. That’s a real but relatively small change for most people, and newer classes of biologics appear to have little to no effect on weight at all.
How Much Weight Gain to Expect by Drug Type
Not all biologics affect weight equally. Among the TNF blockers, which are the oldest and most studied class, etanercept and adalimumab showed the most weight gain in a systematic review published in Frontiers in Pharmacology, averaging about 2.3 kilograms each. Infliximab came in lower at around 0.9 kilograms. These averages mask a wide range of individual experiences: the standard deviations were large (around 5 kilograms in either direction), meaning some people gained significantly more while others lost weight or stayed the same.
Newer biologics that target different parts of the immune system tell a different story. In a head-to-head comparison of psoriasis patients followed for seven months, those on infliximab gained nearly 3 kilograms on average, while patients on secukinumab (which blocks a different immune signal called IL-17) and ustekinumab (which blocks IL-23 and IL-12) showed essentially no weight change at all. If weight gain is a concern for you, it’s worth knowing that the specific biologic your doctor recommends matters more than the category “biologics” as a whole.
One small study of ustekinumab in psoriasis patients did find that 64% gained weight over 28 weeks, averaging about 2.3 kilograms. But with only 11 patients in that study, it’s hard to draw firm conclusions, and the larger comparative data suggests IL-17 and IL-23 blockers are largely weight-neutral.
Why TNF Blockers Affect Weight
The connection between TNF blockers and weight gain makes biological sense once you understand what TNF does in the body. TNF is an inflammatory molecule, but it also acts as a metabolic accelerator. It triggers the release of stress hormones like cortisol, epinephrine, and glucagon, which ramp up your metabolism and break down muscle protein. In people with active inflammatory disease, chronically elevated TNF contributes to muscle wasting and unintentional weight loss, a process called cachexia.
When a TNF blocker suppresses that signal, several things shift at once. Your body stops burning through muscle tissue at an accelerated rate. Your appetite often improves. Your insulin sensitivity changes, allowing cells to take up glucose more efficiently. For people who were underweight or malnourished from their disease, this is genuinely a good thing. For people who were already at a stable weight, the same metabolic shift can tip the balance toward gradual fat storage.
There’s also an important nuance around insulin sensitivity. TNF blockers appear to improve how your body handles blood sugar if you’re at a normal weight, but research in rheumatoid arthritis patients found this benefit doesn’t extend to people who are already obese. Enlarged fat tissue produces its own inflammatory signals through pathways that TNF blockers don’t fully address, which may explain why the metabolic benefits of these drugs vary so much from person to person.
Weight Gain vs. Disease Recovery
This is the part that often gets overlooked: for many people on biologics, gaining some weight is actually a sign the treatment is working. This is especially true in Crohn’s disease, where inflammation in the gut directly interferes with nutrient absorption and appetite. In a prospective study of Crohn’s patients on infliximab, BMI increased significantly within the first 10 weeks of treatment. Patients who responded well to the drug gained more weight than those who didn’t respond, and patients who achieved remission gained the most.
That pattern makes it difficult to separate “drug-caused weight gain” from “your body finally absorbing food properly again.” If you’ve been living with active inflammatory bowel disease and you gain weight after starting a biologic, your body may simply be recovering from a period of chronic malnourishment.
What’s Actually Changing: Fat vs. Muscle
The type of weight gained also varies by drug class. A study tracking body composition in rheumatoid arthritis patients found that TNF blockers led to meaningful gains in lean mass over one year, including total lean mass, fat-free mass, and skeletal muscle mass. There was a small increase in subcutaneous fat (the layer just under your skin), but no change in visceral fat, which is the more metabolically dangerous type that surrounds your organs. The net gain in lean tissue was about 1 kilogram, a modest but clinically relevant improvement for patients whose disease had been eating away at their muscle.
Patients on non-TNF biologics showed a different pattern at six months: a slight decrease in lean mass and an increase in body fat percentage, though these changes didn’t persist at the one-year mark. Patients on conventional non-biologic drugs saw an increase in visceral fat over the same period with no improvement in lean mass. So while TNF blockers may cause the number on the scale to go up, the composition of that weight gain appears more favorable than what happens without biologic treatment.
JAK Inhibitors and Weight Gain
JAK inhibitors are a newer class of targeted therapy that are taken as pills rather than injections. They’re not technically biologics, but they’re often discussed alongside them and prescribed for the same conditions. A systematic review covering 16,000 patients found that about 6% reported weight gain on JAK inhibitors overall, with rates varying by specific drug. Ruxolitinib had the highest rate at 12%, followed by upadacitinib at 5% and tofacitinib at 3%.
Patients being treated for skin conditions had lower rates of weight gain (4%) compared to those treated for other conditions like rheumatoid arthritis or inflammatory bowel disease (7%). This likely reflects differences in baseline disease severity and nutritional status rather than the drugs working differently in different tissues.
Keeping Weight Stable on Biologics
If you’re starting a biologic and concerned about weight, the most practical thing you can do is establish a baseline. Know your weight before you begin treatment so you can track real changes rather than guessing. A gain of 1 to 3 kilograms over the first several months is within the expected range for TNF blockers and doesn’t necessarily signal a problem.
Regular physical activity and attention to diet remain the most effective tools for managing any weight shift during treatment. Research on biologic dosing in patients with obesity emphasizes that optimal drug outcomes should be paired with exercise and lifestyle changes that address both inflammatory and non-inflammatory contributors to weight gain. This is standard advice, but it carries extra relevance here because the metabolic changes from biologics are modest enough that normal lifestyle adjustments can realistically offset them.
If you’re gaining more than expected or the gain is rapid, the issue may not be the biologic itself. Reduced pain and improved mobility on a new treatment can change daily routines in unexpected ways, and corticosteroids that are sometimes prescribed alongside biologics are far more potent drivers of weight gain than biologics alone. Sorting out which factor is responsible for what you’re seeing on the scale is worth a conversation with whoever is managing your treatment.