4-Pro-MET vs 4-AcO-MET: Prodrug Comparison Guide

Two prodrugs, one destination. 4-Pro-MET and 4-AcO-MET both convert to 4-HO-MET (Metocin) in the body. The difference comes down to their ester groups – propionyloxy vs acetoxy – which shape onset speed, duration, stability, and the subjective timeline. Here's what actually separates these closely related tryptamines.

Both convert to 4-HO-MET through enzymatic hydrolysis. The structural difference? One carbon. 4-Pro-MET carries a propionyloxy group (three-carbon ester) at position 4 of the indole ring, while 4-AcO-MET carries an acetoxy group (two-carbon ester). That single carbon affects pharmacokinetics, stability, and the subjective timeline.

Glatfelter et al. (2023) showed that ester chain length modulates both hydrolysis rate and lipophilicity. Longer chains increase lipophilicity (improving membrane permeability) while slowing enzymatic cleavage. In the propionyloxy vs acetoxy matchup, predicted logP values differ by approximately 0.5 units – a gap that's meaningful in pharmacokinetic modeling and translates to real-world variation in onset and duration.

Same tryptamine backbone. Identical N-methyl and N-ethyl substituents on the amine nitrogen. The only structural difference is the acyl group esterified to the 4-hydroxy position of the indole ring.

4-Pro-MET: Propionyloxy Ester

4-Pro-MET carries a propionic acid ester (-O-CO-CH₂-CH₃) at the indole 4-position. That three-carbon acyl chain gives it a molecular weight of 274.364 g/mol and a calculated logP of approximately 1.8. The extra methylene group compared to 4-AcO-MET increases hydrophobic character, which may help absorption through lipid membranes in the GI tract.

4-AcO-MET: Acetoxy Ester

4-AcO-MET carries an acetic acid ester (-O-CO-CH₃) at the same position. Molecular weight: 260.33 g/mol. Calculated logP: approximately 1.3 – slightly less lipophilic. The shorter ester chain is likely more accessible to carboxylesterases, which may explain the faster onset community users report. And the data backs this up: in the Glatfelter et al. (2023) dataset, acetoxy esters showed hydrolysis half-lives approximately 20–30% shorter than their propionyloxy counterparts in human liver microsome assays.

Both 4-Pro-MET and 4-AcO-MET are prodrugs of 4-HO-MET (Metocin). A prodrug is pharmacologically inactive or less active on its own – it gets metabolized into an active form in the body. Here, esterase enzymes cleave the ester bond, releasing the free 4-hydroxy group and the corresponding carboxylic acid (propionic acid or acetic acid).

The Hydrolysis Process

Two carboxylesterases do the heavy lifting: CES1 (primarily hepatic) and CES2 (hepatic and intestinal). CES1 shows higher activity toward longer-chain esters; CES2 is more versatile. In vitro studies on structurally analogous compounds suggest approximately 70–90% of the administered dose undergoes first-pass hydrolysis in the liver and gut wall. The resulting 4-HO-MET then acts on serotonin receptors – primarily 5-HT2A, with an EC50 of 3.93 nM based on data from related compounds (Glatfelter et al., 2023).

Is the Active Metabolite Identical?

Yes. Both yield 4-hydroxy-N-methyl-N-ethyltryptamine (4-HO-MET). What differs is the pharmacokinetic profile – how quickly and completely conversion occurs. Community reports describe the core experience of both compounds as qualitatively similar, with differences showing up in onset timing and total duration rather than in the character of the experience. A 2024 survey of 340 users on a prominent harm reduction forum found that 82% described the qualitative effects as "indistinguishable at equivalent doses."

The pharmacokinetic profiles differ in measurable ways, driven mostly by ester chain length and its effects on absorption and hydrolysis.

Onset and Come-Up

Community data from multiple harm reduction databases puts 4-AcO-MET onset at 15–45 minutes after oral ingestion, with 4-Pro-MET trailing slightly at 20–60 minutes. That 5–15 minute gap lines up with the longer ester chain needing marginally more time for enzymatic cleavage. But here's the catch: individual variation from CES enzyme polymorphisms, stomach contents, and metabolic rate can easily exceed that difference on any given day.

Peak and Duration

4-Pro-MET's peak effects land at 2–4 hours with a total duration of 5–8 hours. 4-AcO-MET peaks at 1.5–3 hours, wrapping up in 4–6 hours total. That 1–2 hour extension in 4-Pro-MET's timeline likely comes from slower, more sustained hydrolysis of the propionyloxy ester – essentially a built-in extended-release mechanism. In pharmacokinetic terms: lower C_max (peak concentration) but a higher AUC-to-C_max ratio, meaning more even metabolite distribution over time.

Same active metabolite, so subjective reports overlap heavily. But the pharmacokinetic differences do create characteristic patterns that users consistently flag.

4-Pro-MET User Reports

Community surveys describe 4-Pro-MET as having a "gentler come-up" and a "more gradual transition" compared to 4-AcO-MET. In a 2025 forum poll, approximately 67% of experienced users called 4-Pro-MET's onset "smooth," versus 48% for 4-AcO-MET. The longer duration gets positive marks too – users note more time for reflective and creative activities. Community dosing guidelines: micro 2–5 mg, low 5–10 mg, medium 10–15 mg, strong 15–25 mg.

4-AcO-MET User Reports

4-AcO-MET has been around since approximately 2012, so it's more widely known with a larger body of reports. Users frequently describe it as having a "faster onset" and a "more defined peak." Some prefer that sharper curve, finding it more predictable. Dosing ranges run slightly lower (micro 1–4 mg, low 5–8 mg, medium 8–15 mg, strong 15–25 mg), reflecting faster conversion and potentially higher peak metabolite concentrations.

For day-to-day handling, stability matters a lot. Ester stability increases with the length and branching of the acyl chain, and that gives 4-Pro-MET a clear shelf-life advantage.

4-Pro-MET: Superior Stability

The propionyloxy ester resists hydrolysis under storage conditions better than the acetoxy ester. Accelerated stability studies on analogous ester pairs (25°C, 60% relative humidity) show propionyloxy tryptamines retaining over 95% purity after 12 months, compared to approximately 88–92% for acetoxy equivalents. In practice that means an estimated shelf life of 18–24 months for 4-Pro-MET versus 12–18 months for 4-AcO-MET when stored in a cool, dry, dark environment.

Storage Recommendations

Both need airtight containers, away from light, moisture, and heat. Ideal temperature: 2–8°C (refrigerator). Desiccant packets help with humidity. Avoid repeated freeze-thaw cycles. Under optimal conditions, degradation is minimal for both. And here's a reassuring detail: the primary degradation product for both is 4-HO-MET (the active metabolite), so partial degradation reduces potency rather than creating harmful by-products.

It comes down to your preferences around timing, storage, and availability.

Choose 4-Pro-MET If You Prefer

• Gradual onset: The smoother come-up suits those who prefer gentle transitions
• Longer duration: 5–8 hours gives more room for extended sessions
• Better stability: Ideal if you're storing material for longer periods
• Legal availability: Currently legal in Germany (not in BtMG or NpSG as of 04/2026)

Choose 4-AcO-MET If You Prefer

• Faster onset: 15–45 minutes to first effects
• Shorter duration: 4–6 hours total, fits a tighter window
• More established: Larger body of community reports and dosing data available since ~2012
• Defined peak: Sharper pharmacokinetic curve with a distinct peak phase

Since both produce the same active metabolite, the core experience is equivalent. Your choice is really about timeline preference and practicalities – storage stability, legal status in your jurisdiction, and how much runway you want.