Antipsychotic-induced weight gain is one of the few adverse effects that is entirely measurable, entirely predictable and almost never measured. The reason is mundane: the scales are in another room, the clinic is running late, and the patient is acutely unwell so the weight feels like the least of it.
The consequence is that six months later there is no baseline. A patient who has gained 12 kg is indistinguishable in the record from one who has always been that weight, and the conversation about doing something never starts.
So: weigh at the first prescription, record it, and record a waist circumference too — central adiposity is what carries the cardiometabolic risk and it moves independently of weight. Add a fasting glucose or HbA1c and a lipid profile. Then repeat at three months, which is when antipsychotic weight gain is steepest, and annually after that.
Tell the patient the number and what you expect. A patient warned that this drug commonly causes weight gain, and shown their starting figure, is far more likely to report it than one for whom it arrives as an unexplained change. Unexplained weight gain is a common reason for silent non-adherence, and it is rarely volunteered.
One practical note for a busy clinic: if only one measurement is possible, take the weight. It takes ten seconds, it needs no laboratory, and it is the measurement that anchors everything that follows.
- Weigh the patient at the consultation where the antipsychotic is first prescribed, and write the number down
- Add waist circumference — central adiposity carries the risk and moves independently of weight
- Repeat at three months, when antipsychotic weight gain is steepest, then at least annually
- Warn the patient explicitly and show them their starting number, so a change gets reported rather than hidden
- If only one measurement is possible, take the weight; it anchors every later decision
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