Onset of effects, by domain:

• Libido + spontaneous erections: 1-3 hours post-injection. Often the first thing users notice. Erections can be persistent and inconvenient — multiple hours, sometimes during sleep or in inappropriate contexts. Some users explicitly use this as the desired effect; others find it disruptive.
• Nausea: 30-90 minutes post-injection during loading phase. Frequently severe enough to require antiemetic (ondansetron) or to abandon the protocol. Tolerance to nausea typically develops over 7-14 days as receptors desensitize.
• Facial flushing: within 30-60 min, usually fades over 2-4 hours.
• Tanning: 5-7 days for first visible change at daily loading dose, plateau at 4-6 weeks. UV exposure dramatically accelerates and deepens the effect.
• Mole/freckle darkening: within 1-3 weeks. This is universal, not optional. Existing pigmented lesions (nevi, freckles, areolas, perineum, scrotum, lip vermilion border) darken disproportionately to background skin.
• Appetite suppression: mild-to-moderate, day-of-injection. Some users find this a feature; for an combat athlete eating to support training, it's a bug.

Characteristic effects pattern:

• "Tan that lasts" — user cohorts on maintenance dose 2-3×/week report year-round visible pigmentation with periodic UV exposure, including in winter.
• "Sexual side-effects-as-feature" — many users adopt MTII for the erectile/libido effect, not the tan. This use case overlaps with PT-141 (bremelanotide), the cleaner MC4-leaning analog.
• "Skin signature" — dark spots, freckle multiplication, lip darkening, gum/oral mucosa pigmentation (case-reported), darker areolae and perineum. Some users find this aesthetically desirable (the "model tan"); others find it concerning.

Honest variability:

• MC1R loss-of-function variants (red hair, fair Celtic/Nordic phenotype) get markedly less tanning response, and what they do get may be uneven freckle-multiplication rather than uniform tan. This is not a pharmacology failure — it's that MC1R variants can't fully signal even when activated. the user's Nordic/British ancestry suggests intermediate risk for MC1R variants; 23andMe results (June 2026) will inform.
• Phototype IV-V skin (Mediterranean, South Asian, sub-Saharan African) — less reason to use MTII (already tan, MC1R fully functional, native pigmentation strong); side effects unchanged.
• Dose response is steep and individual — same 500 mcg may be unnoticeable for one user and produce vomiting in another.

• Tolerance to therapeutic effect (tanning): Modest. Most users hit a pigmentation plateau at 4-6 weeks and maintain on 2-3×/week dosing. Going off MTII for several weeks and then restarting still works.
• Tolerance to side effects (nausea, flushing): Develops over 7-14 days. This is receptor desensitization, not whole-body adaptation.
• Cycling: No formal cycling pattern in the gray-market literature. Common pattern is "load → maintenance → seasonal break (winter, when UV is minimal anyway)." Some users dose continuously for years; long-term safety in this pattern is unstudied.
• Reset / discontinuation: Pigmentation gradually fades over 2-3 months without dosing + without UV. Mole darkening may persist longer or partially. Any new nevi formed during use generally do not regress.

Synergistic with

• bremelanotide (PT-141) — Same family, MC4-leaning. Some users stack briefly to combine MTII's tanning durability with PT-141's cleaner on-demand sexual function. Mechanism is redundant; risk profile compounds (additive nausea, flushing, BP effects). Not recommended.
• PDE5 inhibitors (sildenafil, tadalafil) — additive on erection; MTII works centrally, PDE5i works peripherally. Increases priapism risk substantially when stacked. If MTII is being used for sexual function, stacking with PDE5i is the most common protocol that ends in ER visits.

Avoid stacking with

• Other melanocortin agonists (afamelanotide, setmelanotide, bremelanotide chronic) — receptor saturation + additive side effects. Pick one mechanism.
• Stimulants (high-dose caffeine, modafinil, amphetamine) — additive sympathomimetic load on the cardiovascular system. The reported rhabdo case involved stim-pattern stacking.
• MAOIs / serotonergic stacks — theoretical concern given some 5-HT involvement in MC pathways; case reports thin but mechanism plausible.

Neutral / safe co-administration

• Standard supplement stacks (omega-3, magnesium, creatine, B-complex, NAC) — no documented interaction.
• the user's V stack base stack — no documented interaction.
• Modafinil — no documented direct interaction, but combined sympathomimetic load + nausea overlap counsels caution.

• Not metabolized via CYP enzymes (peptide; cleared by peptidases + renal excretion). No expected CYP induction or inhibition.
• Hormonal contraceptives: no documented interaction.
• PDE5 inhibitors: functional interaction (additive erection effect → priapism risk).
• Antihypertensives: unstudied; some users report transient BP elevation in early loading phase that could counteract antihypertensives.
• GLP-1 agonists (retatrutide, semaglutide, tirzepatide): theoretical additive nausea + appetite suppression. Worth flagging for users on GLP-1s considering MTII for cosmetic reasons.

• MC1R variants (rs1805007, rs1805008, rs1805009 + ~80 other low-frequency variants) — Loss-of-function variants are the canonical "red hair / fair skin / freckles / poor tanning" SNPs. People with two LOF MC1R alleles produce predominantly pheomelanin (red/yellow, weakly photoprotective) regardless of melanocortin signaling input. MTII cannot rescue MC1R loss-of-function — receptor activation can't drive eumelanin synthesis if the receptor itself is defective. Practical: red-haired Celtic/Irish phenotype users get minimal tanning benefit from MTII, while still getting full nausea + libido + dermatologic risk.
• MC4R variants — affect appetite/satiety/sexual function response magnitude. LOF MC4R variants cause syndromic obesity; MC4R agonists like setmelanotide are FDA-approved for these. MTII response in MC4R variant carriers is largely unstudied.
• For the user (23andMe results June 2026): MC1R genotype will inform the efficacy question. Nordic/British ancestry has ~10-20% MC1R variant carrier rate. If 23andMe shows two LOF MC1R variants, MTII is mechanistically dead for tanning purposes. If zero variants, full tanning response expected. This is one of the few compounds where 23andMe results meaningfully shift the verdict.

COA / vendor reality check

• The "research chemical" framing is a regulatory loophole, not a quality assurance label. As Gerstman 2024 documents in detail, most US "peptide brands" are repackagers of Chinese bulk peptide. Some demand and publish third-party purity COAs (HPLC + mass spec); most don't.
• For a peptide where mis-dosing or contamination has documented serious consequences (rhabdo, priapism, PRES), the gray market quality variability is itself a risk factor.

Baseline (before starting — non-optional if using)

• Total-body skin photography (dermatology-grade or smartphone with consistent lighting, mapped to anatomic regions).
• Dermoscopy of any nevus >2 mm or with prior atypia.
• Family history of melanoma or dysplastic nevus syndrome documented.
• MC1R genotype (23andMe gives you rs1805007, rs1805008, rs1805009 + others).
• Blood pressure baseline (multiple readings over 7 days).
• Basic metabolic panel (renal function — case reports of renal infarction).

During use

• Daily symptom log first 2 weeks: nausea severity, flushing, erection pattern, headache, dizziness.
• Monthly skin self-exam comparing to baseline photos.
• 3-month dermatology follow-up for full-body re-exam.
• CK if any muscle pain + dark urine (rhabdo rule-out).
• BP if symptomatic (PRES has BP correlate; transient hypertension reported).

Post-cycle (if cycled)

• Repeat dermatology exam at 3 months post-cessation to document any nevus changes that emerged late.
• Pigmentation and new nevi may not regress fully — baseline is permanently altered.

• bremelanotide.md — MC4-leaning FDA-approved metabolite of MTII, cleaner for sexual function indication. For the user, the appropriate substitute if sexual function ever becomes a goal.
• retatrutide.md — Triple-agonist GLP-1/GIP/glucagon. Unrelated mechanism but shares "metabolic / aesthetic peptide" gray-market category. Cross-linked for navigation.
• (Future: afamelanotide.md, setmelanotide.md if added — MC1-selective and MC4-selective FDA-approved family members.)