# Melanotan 2 Research: Appetite, Weight, Pigmentation & Sexual Function > Melanotan 2 research, summarized: the appetite and metabolic animal data, the pigmentation studies, the human sexual-function trials, and pharmacokinetics. Cited. The appetite and metabolic literature first, then pigmentation, sexual function, and pharmacokinetics — each finding tied to its source. ## Before the details Most of what we know about Melanotan 2 research comes from animals, not people. The appetite and metabolic work — the focus of this page — is almost all in mice and rats, where the peptide reliably cuts eating and body fat. The human evidence is limited to a handful of tiny, decades-old studies on tanning and on erectile function, with fewer than two dozen people total. There is no large modern trial. That shape matters. Strong, repeated animal findings tell you a molecule does something real in a mouse; they do not prove the same dose is safe or effective in a person. Below, each finding is labeled with the species it came from. Doses are reported exactly as the experiments used them — they are facts about studies, not recommendations, and Melanotan 2 is not approved for human use. ## Appetite and food motivation The cleanest appetite finding comes from the brain's reward circuitry. In male mice, injecting the peptide directly into a reward region called the nucleus accumbens (at 0.1 to 1 nanomole per side) sharply reduced both how much food the mice ate and how hard they worked to earn food — and it did so without making them feel sick (no taste aversion) and without changing their metabolic rate [5]. In other words, the peptide dialed down the *wanting* of food, not just the eating of it. This sits on top of a large body of hypothalamic work showing the peptide suppresses appetite through the MC4R receptor — one of the body's master 'stop eating' switches. The effect is potent enough that loss of MC4R signaling is itself a known cause of obesity, while activating it pharmacologically does the opposite. ## Body weight, fat, and thermogenesis The weight data are where the animal literature is most striking. In obese Zucker rats, repeated central dosing produced robust, sustained reductions in body weight and body fat. The notable twist: tolerance developed to the *acute* appetite-suppressing effect, yet the fat loss persisted anyway — meaning long-term fat loss does not depend on permanent appetite suppression [7]. A second mechanism explains why. Chronic activation of the central melanocortin system reduced body mass in rats *without* requiring long-term calorie restriction; it cut intra-abdominal (visceral) fat and increased the heat-producing capacity of brown adipose tissue [8]. So the peptide drives weight loss by two routes at once — eating less, and burning more. In diet-induced obese mice, subcutaneous dosing reduced body weight and fat, with the data indicating the effect was driven primarily by reduced food intake, acting on both subcutaneous and visceral fat [9]. ## Glucose, insulin, and metabolic signaling The metabolic effects reach beyond fat. When delivered into the brain of mice, the peptide increased insulin sensitivity of glucose disposal and significantly raised skeletal-muscle Glut4 expression — Glut4 being the transporter muscle uses to pull sugar out of the blood [10]. This points to a central (brain-driven) melanocortin influence on whole-body glucose handling, independent of any effect on pigment. The thermogenic side has been probed genetically too. In mice lacking the signaling peptide PACAP — which have impaired heat production — Melanotan 2 partially rescued their thermogenic capacity and restored cold-induced adaptive thermogenesis, placing melanocortin signaling downstream of PACAP in the body's heat-generating pathway [11]. ## Pigmentation and Melanotan 2 tanning The pigmentation work is what made the molecule famous, and it is the basis of all melanotan 2 tanning interest. In the first human pilot — a single-blind, placebo-controlled Phase I study in three healthy men — subcutaneous dosing increased pigmentation on the face, upper body, and buttocks in two of three subjects after only five low doses, with no UV exposure required [1]. The tan comes from MC1R activation pushing pigment cells to make more eumelanin, the darker, more protective form of melanin. A key pharmacology lesson comes from the closely related linear analog (Melanotan I). In a human study, that peptide cleared from the blood within hours, yet skin pigmentation persisted for weeks afterward [35]. Pigment outlasts the peptide because the peptide only *starts* the melanin-making process; the cells keep producing pigment long after the drug is gone. The same principle is why a melanotan tan does not switch off the moment dosing stops. ## Sexual function The sexual effects were discovered by accident during the tanning pilot and then tested directly. In a double-blind, placebo-controlled crossover study in 10 men with psychogenic erectile dysfunction, a single 0.025 mg/kg subcutaneous dose produced clinically apparent erections in 8 of 10 men; the mean duration of firm (>80% rigidity) erection was 38.0 minutes with the peptide versus 3.0 minutes with placebo (p=0.0045), with transient nausea, stretching, and yawning that needed no treatment [2]. The effect is central — it acts on the brain's MC4R-driven sexual circuits rather than on blood flow, which is why it works independently of vascular causes. The pathway is not male-specific. In female rats primed with estrogen and progesterone, intravenous dosing (1 and 3 mg/kg) increased proceptive (solicitation) sexual behaviors without affecting the reflex posture of lordosis — supporting the melanocortin pathway as a target for female sexual motivation [37]. This receptor biology is what a separate, approved sexual-function drug from the same family was built to exploit. ## Pharmacokinetics and the lineage How the body handles the peptide has been studied mostly in animals. A rat study comparing analytical methods after intravenous dosing showed rapid, biphasic, multi-compartment clearance from plasma [36]. No validated human half-life has ever been published for Melanotan 2 itself; the closest human data come from the linear analog, which had an absorption half-life of roughly 0.07–0.79 hours and a beta-phase half-life of roughly 0.8–1.7 hours after subcutaneous dosing, with no detectable levels after oral dosing [35]. A historical review ties the family together: the linear analog (Melanotan I) and the cyclic Melanotan 2 were both patented and tested clinically — the linear one for tanning, the cyclic one for male erectile dysfunction — and a further Melanotan-2-derived analog advanced toward approval for sexual dysfunction in both sexes [3]. The differing effects of each come down to which melanocortin receptors each one prefers. --- A dark-mode reading of the Melanotan 2 record that leads with the appetite and metabolic studies — the robust rodent fat-loss data set in plain type, the tiny old human trials and the documented melanoma, kidney, and priapism harms kept in full view, and the community reports pinned to one side as anecdote; no clinic behind the name despite the 'Rx', and nothing here dosed, sourced, prescribed, or sold.