How do I distinguish drug-attributable fatigue from an energy deficit?

No other medication changes, no dietary changes, nothing else obviously confounding.

I noticed this today and I have not touched anything since, in case the state is diagnostic.

I have not discarded anything yet, so a test is still possible if that is the recommendation.

Should I be treating this as a failure or as noise?

If it persists at an adequate intake, it needs blood work rather than more speculation.

A deficit beyond about a thousand kilocalories a day reliably produces fatigue, reduced training performance and reduced spontaneous movement. With appetite suppressed, deficits of that size arrive by accident rather than by plan.

Sleep quality often changes during rapid weight loss in both directions, and sleep debt produces fatigue that no amount of dietary adjustment will fix.

Sleep restriction produces measurable decrements in subjective energy and in training performance independently of energy intake.

Nothing here is medical advice.

Sleep is a variable here too, and no amount of eating fixes sleep debt.

Start with the actual intake, because a substantially suppressed appetite produces deficits far larger than intended and fatigue is the first symptom.

Three days of honest weighed intake usually settles this. Estimated intake in this situation is systematically wrong and usually wrong in the direction that hides the problem.

It helps to be literal here: fatigue that appeared abruptly, rather than gradually with the deficit, is a different pattern and points away from intake.

Self-reported intake underestimates measured intake by twenty per cent or more in doubly labelled water comparisons.

Research-use compounds are not approved for human use.

Check fluid and sodium before anything more exotic.

edited 22 Mar 2025 by rhian_prydderch — added a caveat about sampling

The underlying point is that dehydration and low sodium intake produce fatigue that is indistinguishable subjectively and much easier to fix.

Hypothyroidism, sleep apnoea, depression and anaemia all present as fatigue and all become more likely rather than less in this population, so attribution to the compound should be a diagnosis of exclusion.

Energy deficits above roughly a thousand kilocalories a day are associated with measurable reductions in resting metabolic rate, spontaneous activity and subjective energy in controlled studies.

Weigh three days of intake honestly. That answers this most of the time.

edited 21 Apr 2025 by cal_hennessy — corrected a unit error in the worked example

The short version: check intake, check hydration, check iron and thyroid if it persists, and only then attribute it to the compound.

Dehydration from reduced fluid intake alongside reduced food intake produces headache, lethargy and postural dizziness, and is the cheapest thing on the list to correct.

The pooled gastrointestinal adverse-event rates across the STEP programme and the SURMOUNT programme are reported in the primary publications and in the FDA and EMA assessment reports, and the assessment reports are more useful because they give the placebo-arm rates alongside the active-arm rates in the same table.

Abrupt onset points away from the deficit and towards something else.