Several common supplements can cause weight gain, though the mechanisms vary widely. Some add pounds through water retention or extra calories, others shift your hormones, and a few may increase your appetite. Understanding which supplements carry this risk helps you figure out whether the number on your scale reflects actual fat gain or something less concerning.
Creatine and Water Weight
Creatine is one of the most popular fitness supplements, and it’s also one of the most reliable scale-movers. The weight you gain from creatine isn’t fat. It’s water. Creatine is an osmotically active substance, meaning it pulls water into your cells wherever it’s stored. Since about 95% of your body’s creatine sits inside muscle tissue, supplementing with it causes your muscles to hold more fluid.
During a typical loading phase of 25 grams per day for a week, total body water increases by roughly 1.4 liters, with body mass rising by about 0.75 kilograms (a little over 1.5 pounds). That water gain is almost entirely intracellular, meaning it’s inside the muscle rather than bloating you under the skin. Over longer-term use at maintenance doses of 3 to 5 grams daily, some people gain 2 to 4 pounds total. If you stop taking creatine, the water weight drops off within a couple of weeks.
Mass Gainers Pack Serious Calories
This one is by design, but it catches some people off guard. Mass gainer supplements are engineered to deliver an enormous number of calories per serving, often rivaling an entire meal or two. A single serving of Optimum Nutrition Serious Mass contains 1,260 calories and 251 grams of carbohydrates. Dymatize Super Mass Gainer sits at 1,280 calories. Even a lower-calorie option like Iron Vegan Athlete’s Gainer delivers 720 calories per scoop.
The calories come overwhelmingly from carbohydrates, with most products packing 120 to 250 grams of carbs per serving. If you’re drinking these on top of your normal meals without burning the extra energy through intense training, the caloric surplus will translate directly into fat gain. People sometimes treat mass gainers like regular protein shakes, not realizing a single serving can contain more calories than a fast-food combo meal.
DHEA and Hormonal Shifts
DHEA (dehydroepiandrosterone) is a hormone precursor sold over the counter and marketed for anti-aging, energy, and hormone balance. It’s produced by the adrenal glands and serves as a building block for testosterone and estrogen. Supplementing with it can meaningfully alter your body composition.
Research has found that higher DHEA levels predict increases in fat mass, skeletal muscle mass, and abdominal fat. In one study, baseline DHEA levels correlated with gains in both subcutaneous and deeper abdominal fat over six months. Part of this may be a direct hormonal effect, since DHEA concentrations are highest in the brain and could influence appetite and fat storage centrally. There’s also an indirect pathway: elevated DHEA often tracks alongside higher cortisol levels, and cortisol is well known to stimulate appetite, particularly cravings for high-fat, sweet foods. The combination of hormonal shifts can slowly push weight upward, especially around the midsection.
Zinc and Appetite Stimulation
Zinc supplements are commonly taken for immune support, but zinc plays a direct role in appetite regulation. It interacts with ghrelin, the hormone that signals hunger to your brain, and it influences taste perception. When zinc levels are low, people often experience reduced appetite and even taste disorders. Correcting a deficiency with supplements can reverse that suppression, leading to noticeably increased hunger and food intake.
This effect is most pronounced in people who were genuinely zinc-deficient to begin with. If your zinc levels are already adequate, supplementing is unlikely to ramp up your appetite. But for people recovering from illness, older adults, vegetarians, or anyone with marginal zinc status, starting a zinc supplement can make food more appealing and meals larger, which over time leads to weight gain.
Melatonin Likely Doesn’t Cause Gain
Melatonin is worth addressing because many people worry about it. The evidence actually points in the opposite direction. Animal studies consistently show that melatonin supplementation reduces body weight, decreases fat deposition, and improves markers like glucose and insulin levels, particularly in animals fed high-fat diets. A meta-analysis of human trials also found a small but significant reduction in body weight with melatonin use.
That said, results in humans are inconsistent, and the effects seem to vary depending on dose, duration, and the population studied. There’s no strong evidence that melatonin causes weight gain. If anything, its influence on fat metabolism and energy expenditure leans slightly favorable.
Multivitamins and Weight
Some people suspect their daily multivitamin is contributing to weight gain, but research suggests the opposite. In a cross-sectional study, men who regularly used vitamin and dietary supplements had lower body weight, less fat mass, and lower BMI compared to non-users. Women who took supplements reported less hunger and fewer episodes of disinhibited eating. In a randomized controlled trial, women taking a multivitamin during a weight-loss program experienced reduced appetite ratings compared to those on a placebo.
Correcting micronutrient deficiencies may actually support a more efficient metabolism. If you’ve noticed weight gain after starting a multivitamin, the timing is likely coincidental rather than causal.
Adulterated Supplements With Hidden Steroids
One underappreciated risk comes from supplements that contain undeclared pharmaceutical ingredients. Some products marketed as “natural” joint or pain relief supplements have been found to contain hidden glucocorticoids, a class of steroid that causes weight gain, fluid retention, and fat redistribution when taken regularly. The largest documented case series identified 12 patients who developed Cushing syndrome or adrenal insufficiency after taking over-the-counter arthritis remedies like Artri King, Ardosons, and Ajo Rey, none of which listed steroids on their labels.
These products are often purchased online, in community stores, or imported from other countries. They appeal to people looking for alternatives to prescription medications. The FDA issued warnings about Artri King in 2022, but enforcement remains limited. If you’re taking a supplement for pain relief and experiencing unexplained weight gain, a round face, or easy bruising, hidden steroids could be the cause. Bringing the product to your doctor for evaluation is a reasonable step.
How to Tell What’s Causing the Gain
The type of weight gain matters. Creatine adds water weight that shows up within the first week or two and stabilizes. Mass gainers add real body mass from excess calories. Hormonal supplements like DHEA shift body composition gradually over weeks to months, often adding both muscle and fat. Appetite-stimulating supplements like zinc work indirectly by making you eat more.
If you’ve recently started a new supplement and noticed the scale climbing, track the timing. Rapid gain in the first week points to water retention. Gradual gain over several weeks suggests either increased calorie intake or hormonal changes. Checking the calorie content on your supplement label is also worth doing, since some protein powders, recovery drinks, and mass gainers carry far more energy than people expect.