Opioids, cannabis, anabolic steroids, and benzodiazepines are the illegal or commonly misused drugs most closely linked to weight gain, though the mechanisms differ sharply from one substance to the next. Some promote fat storage directly, others ramp up appetite or shift food preferences toward sugar, and a few add body mass through muscle tissue rather than fat. Perhaps more surprising is that stimulants like methamphetamine and cocaine, famous for suppressing appetite during active use, often trigger substantial weight gain once a person stops using them. The relationship between illicit drugs and body weight is rarely as simple as “drug in, pounds on,” and the timing of use, cessation, and treatment all matter.
Opioids and Sweet Cravings
Heroin, illicitly obtained fentanyl, and misused prescription opioids share a well-documented tendency to shift food preferences toward sugar and refined carbohydrates. Research has linked opioid use, both short-term and long-term, with increased preference for sweet tastes, weight gain, disrupted blood sugar regulation, and dental problems tied to high sugar intake.1PubMed Central. The relationship between opioid and sugar intake: review of evidence and clinical applications The mechanism involves the brain’s endogenous opioid system, which plays a role in how rewarding food tastes. When you flood that system with external opioids, the hedonic response to sweet and calorie-dense foods gets amplified.
During active heroin or fentanyl use, though, the picture is mixed. Many people using opioids heavily are malnourished and underweight because the drug itself becomes the priority, meals get skipped, and the chaotic lifestyle of addiction crowds out regular eating. Weight gain tied to opioids becomes most visible and clinically significant when people enter treatment.
Methadone, Buprenorphine, and Treatment-Related Weight Gain
Methadone maintenance therapy is one of the most effective treatments for opioid use disorder, but it comes with a well-known side effect: significant weight gain. A study tracking patients after they started methadone found that average BMI climbed from about 27 to 30, translating to roughly an 18-pound increase across the patient group. Women were hit harder, gaining around 28 pounds on average compared with about 12 pounds for men.2PubMed Central. Increases in body mass index following initiation of methadone treatment That kind of increase pushes many patients from the overweight category into obesity.
A meta-analysis looking at longer-term data found BMI increases ranging from about 2 to 5 BMI points after at least a year on methadone, though this analysis did not find a statistically significant sex difference in the magnitude of gain. The same analysis looked at buprenorphine, the other major medication for opioid use disorder, and found no significant weight change after 12 weeks of treatment.3Drug and Alcohol Dependence. Sex differences in weight gain during medication-based treatment for opioid use disorder: A meta-analysis and retrospective analysis of clinical trial data So methadone appears to be the bigger driver here, not opioid treatment in general.
When researchers compared people maintained on buprenorphine with those on methadone, both groups had high rates of overweight and obesity, with over 40% of each group classified as obese. Both groups also showed signs of insulin resistance, with methadone patients showing somewhat more severe metabolic disruption.4Scientific Reports. Metabolic and Addiction Indices in Patients on Opioid Agonist Medication-Assisted Treatment: A Comparison of Buprenorphine and Methadone The weight gain is real and clinically meaningful, and it is one reason why treatment providers increasingly monitor metabolic health alongside addiction outcomes.
Cannabis and the Appetite Paradox
Cannabis has promoted eating for centuries. Users consistently report intense food cravings and heightened enjoyment of food’s taste and smell, a phenomenon so well known it has its own slang.5PubMed. Cannabinoids and appetite: food craving and food pleasure Lab studies confirm that cannabis alters hormones involved in hunger. In a controlled experiment comparing cannabis to placebo, cannabis administration raised levels of ghrelin (the hormone that signals hunger) and lowered GLP-1 (a hormone involved in feeling full). Cannabis also blunted the normal insulin spike that follows eating.6Translational Psychiatry. Effects of oral, smoked, and vaporized cannabis on endocrine pathways related to appetite and metabolism: a randomized, double-blind, placebo-controlled, human laboratory study
So cannabis makes you hungrier, reduces satiety signals, and blunts insulin response. You would expect regular users to weigh more. They do not. A meta-analysis pooling data from multiple large studies found that cannabis users had a mean BMI of about 26 compared with 27.5 for nonusers. Among the heaviest or longest-term users, BMI dropped further to around 25.5, right at the boundary of what is considered a healthy weight. Nonusers, on average, were overweight. The effect size was large and clinically meaningful.7PubMed Central. Theoretical Explanation for Reduced Body Mass Index and Obesity Rates in Cannabis Users
Researchers have floated several explanations for this paradox. One is that cannabis may act differently depending on a person’s starting weight, perhaps functioning as a kind of metabolic regulator that increases intake in underweight individuals without doing the same in heavier people. Another possibility is that the brain’s reward circuitry treats cannabis and food as partial substitutes, so heavy cannabis users may derive enough pleasure from the drug that they eat less overall despite acute munchies episodes. Confounding factors also play a role: cannabis users are more likely to use other substances, tend to be younger, and may differ from nonusers in ways that affect weight independently.8PubMed Central. Marijuana and body weight The honest summary is that cannabis reliably increases appetite in the short term but does not appear to cause weight gain over time in most users. The evidence is genuinely puzzling, and no single explanation has won the field over.
Stimulants During Recovery
Methamphetamine and cocaine are powerfully appetite-suppressing during active use, which is why dramatic weight loss is one of the visible signs of stimulant addiction. But what happens when a person stops is almost the mirror image. Animal research modeling amphetamine cessation found that rats given chronic amphetamine and then taken off the drug gained significantly more weight and ate significantly more food over a 30-day period than control animals that had never received the drug.9PubMed Central. Food consumption and weight gain after cessation of chronic amphetamine administration The brain’s appetite-regulating systems appear to overshoot in the other direction once the suppressive drug is removed.
In humans, the rebound is well recognized in clinical practice. People in early recovery from stimulant addiction often gain weight rapidly, and the gain can be substantial enough to create new health problems. A study of people in addiction recovery found that those with a stronger general propensity for addictive behavior had significantly higher odds of reporting weight gain or increased appetite during recovery.10PubMed Central. Explaining Excessive Weight Gain during Early Recovery from Addiction The researchers tested whether cravings for the original drug were being directly redirected toward food (what some call “addiction transfer”) and found no evidence for that specific mechanism. Instead, the data modestly supported the idea that some people have a general propensity for addictive patterns, and when the drug is removed, appetitive processes that were being suppressed during active addiction rebound on their own.
This matters practically because weight gain during recovery from meth or cocaine is extremely common, often distressing, and occasionally cited by people as a reason they relapse. Treatment programs that anticipate the weight gain and build in nutritional support tend to have better outcomes than those that ignore it.
Anabolic Steroids
Anabolic-androgenic steroids are illegal to use without a prescription in most countries and are widely trafficked for bodybuilding and athletic performance. They cause weight gain, but primarily through increased lean body mass rather than fat. A review of studies on athletes found that steroid use produced strength gains of about 5 to 20 percent and body weight increases of roughly 2 to 5 kilograms, driven by muscle hypertrophy and the formation of new muscle fibers.11PubMed. Effects of androgenic-anabolic steroids in athletes
The weight gain persists even after the drugs are discontinued, at least for a while. A study of long-term steroid-using bodybuilders measured three months after stopping found that they still had greater body weight, more lean body mass, and larger muscle circumferences than non-using bodybuilders.12PubMed. Body composition, cardiovascular risk factors and liver function in long-term androgenic-anabolic steroids using bodybuilders three months after drug withdrawal Anabolic steroids also cause water retention, which can add several pounds of weight that disappears relatively quickly after cessation. The distinction matters: this is weight gain, but it is a different kind from the metabolic disruption caused by opioids or the fat accumulation seen during stimulant recovery.
Benzodiazepines and Food Intake
Benzodiazepines like alprazolam (Xanax), diazepam (Valium), and clonazepam (Klonopin) are prescription medications that are widely misused and diverted. They reliably increase food and fluid consumption. Research has shown that this overeating effect is blocked by opioid antagonists, suggesting that benzodiazepines drive appetite by activating the brain’s own endogenous opioid system, the same reward pathway involved in opioid-driven sweet cravings.13Life Sciences. Minireview Benzodiazepine-opiate antagonist interactions in relation to feeding and drinking behavior
For people who misuse benzodiazepines chronically, the appetite-stimulating effect compounds with the sedation the drugs produce. You eat more and move less, which is a straightforward formula for gaining weight. Benzodiazepines also impair judgment and reduce inhibition, which can lead to binge eating in people who might otherwise exercise restraint. The weight gain is typically gradual rather than dramatic, but over months or years of heavy use it adds up.
GHB
Gamma-hydroxybutyrate (GHB), sometimes called “liquid ecstasy” despite having no chemical relationship to MDMA, occupies an unusual niche. It is a Schedule I controlled substance in the United States and is used recreationally for its euphoric and sedative effects. In the bodybuilding community, GHB has been used specifically because it triggers a burst of growth hormone release shortly after ingestion. It also increases slow-wave (deep) sleep, and since growth hormone is naturally released during deep sleep, these two effects reinforce each other.14PubMed Central. Toxicological Characterization of GHB as a Performance-Enhancing Drug
Paradoxically, though, the metabolic picture for GHB is more complicated than bodybuilders assume. Animal research has shown that GHB-treated mice actually gained less weight than untreated controls, and the drug has been associated with weight loss in treated patients rather than gain.15PubMed Central. Central and peripheral metabolic changes induced by gamma-hydroxybutyrate The growth hormone spike is real but brief, and whether it translates into meaningful muscle gain in the way users hope is far from settled. GHB is a drug where the popular reputation for building mass may outrun the actual metabolic evidence.
Ketamine and Chronic Wasting
Ketamine, increasingly popular as both a party drug and a treatment for depression, pushes body weight in the opposite direction from most of the substances discussed here. Chronic ketamine abuse has been linked to severe gastrointestinal damage, liver dysfunction, and kidney injury. Case reports describe patients presenting with extreme wasting and cachexia after prolonged heavy use.16PubMed Central. Multiorgan dysfunction related to chronic ketamine abuse The weight loss here is not a cosmetic side effect but a sign of serious organ damage. If you are losing weight rapidly while using ketamine frequently, that is a medical emergency, not a bonus.
Synthetic Cannabinoids
Synthetic cannabinoids (sold under names like K2 and Spice) are chemically distinct from plant-derived cannabis and far less predictable in their effects. Their impact on appetite and weight depends heavily on prior cannabinoid exposure. In cannabinoid-naive users, synthetic cannabinoids tend to suppress appetite. But in people who have previously used THC, exposure to synthetic cannabinoids has been reported to stimulate appetite instead, mirroring the munchies effect of natural cannabis. A case report documented a patient using K2 specifically as an appetite stimulant, and rat models have replicated this pattern of THC-primed appetite stimulation from synthetic cannabinoids.17PubMed Central. Self-reported Use of K2/Spice for Appetite Stimulation: A Case Report Whether this translates to meaningful long-term weight gain is unknown, because long-term studies on synthetic cannabinoid users barely exist. The bigger health concern with these substances is unpredictable toxicity, not body weight.
Why Recovery Itself Drives Weight Gain
Across many different drug classes, the period after quitting is when weight gain peaks. Part of this is simple physiology: stimulants that suppressed appetite are gone, opioids that disrupted eating patterns are replaced by stable meals, and a body under chronic chemical stress starts restoring its energy reserves. But part of it is behavioral. Sleep disruption, which is nearly universal in early recovery, has its own metabolic consequences. Animal research on chronic sleep restriction found that sleep-deprived rats consumed 30 to 60 percent more food relative to their body weight than well-rested controls, with females showing the larger increase.18Sleep. The effects of chronic sleep restriction on the hypothalamic-pituitary-adrenal axis and its interaction with abstinence from opioid use Recovery from addiction almost always involves weeks or months of poor sleep, which may independently push calorie intake up.
Stress hormones add another layer. The hypothalamic-pituitary-adrenal axis, which governs cortisol and the body’s stress response, gets dysregulated both by chronic drug use and by the sleep deprivation that follows withdrawal. Elevated cortisol promotes fat storage, particularly around the midsection. For people in early recovery who are already struggling with mood, cravings, and disrupted sleep, the metabolic deck is stacked toward weight gain from multiple directions at once.
Sex Differences in Drug-Related Weight Changes
Women appear to gain more weight than men in several drug-related contexts. The methadone data showing roughly 28 pounds of gain in women versus 12 in men is one example.2PubMed Central. Increases in body mass index following initiation of methadone treatment Broader evidence from antipsychotic medications, which share some pharmacological overlap with drugs of abuse in how they affect dopamine and serotonin systems, shows a similar pattern. In the majority of clinical studies, women on antipsychotics gain more weight and are more likely to develop metabolic syndrome than men. Interestingly, men still show glucose dysregulation and metabolic disruption even when they do not gain as much visible weight, suggesting the metabolic damage can be partly hidden in male patients.19PubMed Central. Preclinical and Clinical Sex Differences in Antipsychotic-Induced Metabolic Disturbances: A Narrative Review of Adiposity and Glucose Metabolism
These sex differences likely reflect a combination of hormonal factors, differences in body composition, and differences in how fat is stored and metabolized. For women entering treatment for substance use disorders, the likelihood of significant weight gain is something worth discussing with a provider upfront, not because it should discourage treatment, but because anticipating it allows for better planning around nutrition and exercise during a vulnerable period.