Keeping a side-effect diary
The instinct is to write down the worst day. The worst day is the least useful entry you will make.
Most people start a side-effect diary on a bad day. That is the natural moment to reach for your phone — something is happening, it is unpleasant, and writing it down feels like doing something about it.
It also produces the least useful record you could keep. A diary made only of bad days describes a worse fortnight than the one you actually had, and it leaves out the two things a prescriber will reach for first.
The pattern is the information, not the incident
Here is the problem with any single entry. A side effect that is fading and a side effect that is entrenched look identical on a given Tuesday. Both are “felt sick this morning.” Only one of them changes what happens next.
What separates them is the shape over two or three weeks — is this happening less often, less severely, later in the day, or exactly as much as it was? You cannot see that from inside a bad morning. You can see it instantly from six weeks of short entries.
That is the whole case for keeping one. Not diligence, not thoroughness — just that the useful signal only exists across time, and memory does not store it that way.
Four fields, and one that people miss
What it is, in plain words. “Queasy for about an hour after taking it” beats “GI upset.”
When in the day. Morning, with food, two hours after the dose, overnight. Timing is often what makes something solvable.
How bad, on your own scale. Any scale works — one to five, mild/moderate/bad. Consistency matters far more than accuracy. A scale you apply the same way every time is useful even if it is completely idiosyncratic; a carefully calibrated one you keep rethinking is not.
What it stopped you doing. This is the field people leave out and the one a prescriber can act on most directly. “Tired” is hard to weigh. “Too tired to drive to work twice this week” is not.
And the one that carries most of the meaning: log the dose change on the same timeline. Nearly every question you will be asked later is secretly a question about timing. Did this start when the dose went up? Did it settle before or after? Was there anything at the old dose? A diary without dose dates answers none of that; with them, it answers most of it in a glance.
Write down the uneventful weeks
A fortnight where nothing much happened is real data, and it is the entry nobody makes. It is what turns “I have been all over the place” into “it was steady for two weeks after the increase, then it slipped” — which is a completely different conversation.
One line every few days, including the dull ones, beats a detailed paragraph written only when things are bad.
Some effects settle, some do not
Many of the effects people notice in the first days or two weeks ease as the body adjusts. Some do not, and some are worth raising immediately rather than waiting to see. That distinction is not one to make on your own from a diary — it is precisely what the diary is for: bringing an accurate picture to someone qualified to interpret it.
The NHS is explicit that if you are “finding side effects hard to manage,” the answer is to talk to your doctor.1 That is not a last resort after enduring it for a month. It is the intended route.
And a diary is not the right tool for something frightening or severe. If an effect is alarming you, or getting rapidly worse, that is a phone call to your prescriber or urgent care — not an entry to review at your next appointment.
Where to keep it
Anywhere you will actually open. A note on your phone works. Nagi's medication diary has a side-effect field and puts effects and dose dates on one timeline in the doctor report, which is the format that makes the pattern visible — but the habit is doing the work, not the app.
The short version
Record four things briefly and often, including on the days nothing happens, and always alongside the dose dates. The point is not to document your suffering. It is to be able to answer, in one sentence, a question that is otherwise impossible to answer honestly: is this getting better or not?
— Richard MacLean
Related: should I tell my doctor about side effects? and keeping track of medication changes.
Sources
- NHS, Overview — Antidepressants. nhs.uk — accessed 10 August 2026.
Nagi is a mental health companion, not a medical device, and nothing here is medical advice. Never start, stop, or change the dose of a prescribed medication without talking to your prescriber.