Lithium and weight: how to track the side effect honestly

Weight gain is a documented side effect of lithium. It is one of the reasons people stop a medication that is otherwise working. It's a trade-off worth tracking — not a moral test, and not something to ignore.

What is well-established

MedlinePlus lists weight gain among the side effects of lithium that should be reported to a prescriber2024. Mayo Clinic's lithium drug page documents the same2024, alongside thirst, GI effects, and thyroid changes. NIMH includes weight changes among the side effects of mood stabilizers in general2024.

The size of the effect varies — some people see no change, some see significant gain over months — and it interacts with other things: thirst-driven calorie intake (lithium increases thirst), thyroid changes, sleep changes, and other medications. So: weight change on lithium is well-documented, but the size and timeline are individual.

Why tracking helps

Without data, the question "is the medication causing weight gain?" gets answered by the most recent week. With data, it gets answered by trajectory.

Ambulatory assessment in psychiatry routinely shows that periodic logging captures patterns retrospective recall does not2013. The same logic applies here: a chart of weight against the date your dose changed is the difference between guessing and knowing.

What to log

The minimum useful set:

Weight, weekly (not daily — daily fluctuation is noise). Same time of day, same conditions.
Lithium dose as a note when it changes.
Sleep hours every night. Sleep tracks closely with both mood and appetite for many people, so logging it alongside weight makes the picture more useful than weight alone (sleep is also a documented prodrome of mood episodes in bipolar disorder, which is the main reason it is in MoodSync2008).
Mood scales. Without them, you cannot see whether the weight trade-off is buying you the mood stability you started lithium for.

How to read the chart with your clinician

The questions worth raising:

Has the weight curve changed slope since the dose change? A clear inflection point at the same date is informative.
Has the mood pattern changed? Fewer or shorter episodes, fewer hospitalizations, fewer 3-on-depression weeks?
What is the trade-off you are willing to accept?

Smartphone-based monitoring exists for exactly this kind of multivariable question2015: what is mood doing, what is sleep doing, what is the medication regimen doing, all in one view.

What to bring to the visit

A short summary like: "weight has risen X pounds over Y months since starting lithium at dose Z. In the same period, depression scores have moved from a typical Y to a typical X." That sentence gives your clinician something to act on.

How to log this in MoodSync

MoodSync keeps a daily yes/no on whether you took your meds, alongside the four mood scales and sleep. The notes field is where the rest goes — your weekly weigh-in, the date your dose changed, side effects on a given day. The chart you scroll back through at the visit then carries the weight trend, the mood trend, and the dose-change marker in one place.

What this will not replace

This is not pharmacology advice. Do not adjust your lithium dose based on weight changes alone; that is a discussion to have with your prescriber. The point of tracking is to make the discussion specific, not to bypass it.