The blog

Long-form on trend maths, dose tracking and the strange business of measuring a body with a bathroom scale. New post every other day.

• How to read dose markers and level shading on a weight chartPeptrend draws each dose as a dashed line on the weight chart and the in-system estimate as a shaded band. What each mark means, and what not to infer.
• Why your trend line changes last week (LOESS and Savitzky-Golay)Two of Peptrend's trend methods read weigh-ins on both sides of each day, so new readings redraw recent days. On one log, LOESS moved a day 0.62 lb on average.
• Started logging mid-treatment? Why the level estimate reads lowPeptrend's In Your System estimate counts only logged doses. Start a log in month three and it reads half the full figure for a week. How to backfill.
• How many weigh-ins before a weight trend means anything?On one real log, two weeks of readings gave a weekly rate that showed a gain in over a quarter of windows. What four and eight weeks of data change, measured.
• Weekly GLP-1 doses across time zones: the log, curve and reminderFly from Los Angeles to Tokyo and a Sunday-night dose reads as Monday afternoon. What Peptrend's log, level curve, next-dose date and reminder do when you travel.
• Missed or late GLP-1 dose? How to log what actually happenedLog a late dose at the time you took it and leave a skipped one out. What that does to Peptrend's next-dose date, its overdue ring and its level estimate.
• Is a weekly average weight better than a daily weigh-in?A calendar-week mean ignores more scale noise than any of Peptrend's other fourteen trend methods, and says nothing about today. Measured on one real log.
• Switching from semaglutide to tirzepatide: what the dose log showsAfter a switch, Peptrend draws semaglutide clearing on its own 7-day half-life while tirzepatide builds on 5 days. Two lines, never one total, with the numbers.
• Tracking two medications at once in a GLP-1 dose logSince version 1.5.0 Peptrend logs a main medication and any number of others, each with its own half-life, its own curve and its own next-dose date.
• Glapp alternative: oral doses, maintenance goals, free loggingGlapp now sells Glapp Max plans. Peptrend logs doses and weigh-ins free with no account, models daily tablets and supports Maintain goals. Where Glapp still leads.
• MeAgain alternative: dose and weight logging with no accountMeAgain covers food, symptoms, photos and doses; its FAQ counts shot and pill tracking among Premium's features. Peptrend logs doses and weigh-ins free.
• Orforglipron tracker: how the daily tablet is logged and modelledVersion 1.3.3's orforglipron preset came from phase 1 papers; 1.4.0's comes from the Foundayo label. What each models, and how to set all three yourself.
• Oral semaglutide tracker: why a tablet isn't charted as mg takenHow Peptrend logs a daily semaglutide tablet: the Oral route, a 0.8% absorbed share, a 1.5-hour peak, and why the newer 1–2% tablets need a different setting.
• How much is still in your system? A free estimate, explainedPeptrend's In Your System chart is free: a milligram estimate from your logged doses and a published half-life. What it computes, how to read it, where it stops.
• Free GLP-1 tracker apps for iPhone: what each lets you do freeWhat ten GLP-1 and peptide trackers say is free in their own listings, checked 18 September 2026, and exactly which parts of Peptrend are free and which are Pro.
• GLP-1 tracker apps compared by their App Store privacy labelsTwelve GLP-1, peptide and weight apps, compared by what their App Store privacy labels declare. Two read Data Not Collected. Checked 18 September 2026.
• Why active energy falls when your training has not changedActive energy counts the cost of moving your mass, so the same walk costs less as you get lighter. One study found it fell further than that.
• Does a GLP-1 raise resting heart rate? What the trials measuredA meta-analysis of 12 randomised trials in 15,313 adults found GLP-1 receptor agonists raised resting heart rate by 3.47 bpm against placebo.
• Does a GLP-1 change your VO₂ max? What the number measuresVO₂ max is reported per kilogram of body mass, and your watch feeds your weight into the estimate too. Losing weight moves the number on its own.
• What is Athlete Mode? Ten fitness metrics beside your dosesNew in 1.4.0: ten fitness readings from Apple Health, charted beside your logged doses and coloured by the dose in effect. Read-only, opt-in, silent on causation.
• Is Peptrend free? What the free tier covers and what Pro costsYes, there is a free tier. Every weigh-in, dose and vial you log is free, along with the raw weight chart, import, export and iCloud sync. Pro buys the trend line.
• Best weight trend and GLP-1 tracker apps in 2026, comparedNo app found does all three jobs: the GLP-1 trackers log injections, the smoothing apps publish real trend maths, the peptide calculators do mg/mL. Eleven apps.
• Which weight trend method should you use? Fifteen, measuredFor most logs the answer is the shipped default, Robust Adaptive EWMA. The three reasons to override it, and why fifteen methods finish 4.0 lb apart.
• How long is a reconstituted vial good for? The clock starts at mixingThe in-use window on the label runs from the moment powder met water. Peptrend saves that moment to the minute, tags each dose to its vial, and counts the draws.
• Does oral semaglutide have the same half-life as the injection?About a week either way. The interval is what differs: a daily tablet ripples by about 10%, while a weekly injection swings by a factor of two.
• Shotsy vs Peptrend: GLP-1 dose logging and weight trend mathsShotsy is one of the two largest GLP-1 trackers, and its shot log and site rotation are the better half of it. It names no smoothing algorithm. Checked 18 Sep 2026.
• How fast is too fast to lose weight? The 1.5 lb/week capPeptrend refuses any plan faster than 1.5 lb a week and greys out Save. Where that cap sits against published rates, and how the rate, ETA and deficit are computed.
• Weight loss plateau on a GLP-1? Check the trend, not two morningsA flat fortnight is only a plateau once it clears your own noise. On one log, 13 of 67 fortnights showed no loss on the scale while the trend still ended lower.
• How many units is 1 mg? Why a calculator should stop at mg/mLA unit is a mark on a syringe barrel, so 1 mg has no unit count until you name the barrel. Peptrend divides strength by water, prints mg/mL and stops there.
• How to import your weight history from Apple HealthOne tap reads every body-mass sample Apple Health holds, back to the first. Peptrend writes back only the weigh-ins you type in.
• Why the scale hasn't moved in 2 weeks: the signal-to-noise mathsTwo weeks is rarely long enough to tell a real change from ordinary scale noise. Work out your own noise-to-signal ratio, then how many days it takes to clear it.
• When does a GLP-1 peak after injection? Tmax and the absorption curveHours to days after the shot — 1 to 3 days for semaglutide, 8 to 72 hours for tirzepatide by the labels. The peak lands at D × F × 2^(−Tmax ÷ half-life).
• What one bad weigh-in does to your trend line (rolling median)One 10 lb error moves a 7-value rolling median 0.4 lb and a 7-value moving average 1.43 lb. Double the error and the mean's miss doubles; the median's does not.
• Is there a TrendWeight app for iPhone? TrendWeight vs PeptrendTrendWeight is web-only and free forever, takes typed weigh-ins or a Withings sync, and publishes every formula it uses. Peptrend is an iOS app with fifteen methods.
• Injection site rotation: how to keep track without a body mapRotation works when the order is decided in advance. Peptrend saves a site with every dose and suggests the next by round-robin through your list — no body map.
• Why your trend line follows every weigh-in (Hull moving average)Alan Hull's filter subtracts its own lag until the line leads the scale rather than trailing it. On 84 real weigh-ins it ignores 14.3% of daily movement.
• Does Happy Scale log GLP-1 doses? Happy Scale vs PeptrendHappy Scale does not advertise injection or dose logging and does not publish the formula behind its recommended default. Peptrend publishes fifteen and logs doses.
• What your trend does on days you skip a weigh-in (interpolation)On a day you did not weigh, the app draws a straight line between your two nearest readings and hands it to the filter — 142 of 226 days on one user's log.
• Why one trend method sits 4 lb above the scale (Kaufman KAMA)KAMA squares its smoothing constant, capping the alpha near 0.048. On 84 real weigh-ins it ignores 81.3% of daily movement and ends 4.0 lb above the last weigh-in.
• How to chart a titration, and why the scale lags a dose stepPeptrend colours each segment of the weight line by the dose behind it, so a 1.2 to 3.5 mg ladder reads as a four-colour ramp. Why a step takes weeks to land.
• Why your weekly rate changes every day (the Kalman filter)The Kalman method carries your rate of change as part of its state and updates it with every weigh-in, so the weekly figure moves daily. Measured on 84 weigh-ins.
• How much bacteriostatic water? Reconstitution is one divisionThere is no single right amount — the water you add sets the strength. A 6 mg vial with 3.0 mL is 2.00 mg/mL; the same vial with 0.7 mL is 8.57 mg/mL.
• How long until your weight trend catches up? 28 days, or 5An EWMA at α = 0.1 needs 28 days to absorb 95% of a real change. DEMA gets there in 8 days and TEMA in 5 by subtracting an estimated lag, and both overshoot.
• Why your trend line lags behind the scale (SMA vs EWMA)A trend line lags because it averages weigh-ins you already took. At the shipped defaults a 7-day SMA sits 3 days back and an EWMA at alpha 0.10 sits 9 days back.
• How long until a GLP-1 reaches steady state? Four to five half-livesFour to five half-lives — 28–35 days for semaglutide, 24–30 for retatrutide, 20–25 for tirzepatide. Why weekly doses stack, and what 'in your system' counts.
• Why your weight trend reads higher than the scale (Hacker's Diet)A trend line that skips the days you missed takes fewer steps down, so it sits above your weight: 186.0 lb against a last reading of 183.0 on one user's log.
• Can you use trading indicators on your weight? DEMA, HMA, KAMAThe price-only ones, yes. DEMA, TEMA, HMA and KAMA read one number per period, so nothing in them assumes a market. On one real log they finish 4.0 lb apart.
• Why your weight trend barely moves when the scale jumps (EWMA)Your trend line travels a tenth of the way toward each new weigh-in, so a 5.4 lb morning shifts it 0.54 lb. What alpha does, and why a shock halves in 6.6 days.
• What should you log with every GLP-1 dose? Nine fieldsNine typed fields: time, medication, milligrams, route, site, vial, taken, pain and colour. What each one lets you answer later, and why there is no notes box.
• Why did I gain weight overnight? Water, glycogen and gut contentsAn overnight gain is water, glycogen, gut contents and sodium. In one 84-weigh-in log the daily swing ran 11.9 times the real rate of change underneath it.