Weight gain and medication

Generically, if a medication leads to weight gain, the mechanism is one or a combination of these:

* Water retention increase, or perhaps unpredictable fluctuation. It may or may not feel "bloated", it depends on whether the retention is general (over the whole body) or localized (maybe in an internal/non-obvious place).

* Fatigue (perhaps subtle), so moving less (daily life and/or exercise intensity) therefore burning fewer calories. (Fidgeting alone can burn in the low hundreds of calories daily, and noticing reduced fidgeting in oneself is pretty hard.)

* Appetite increase, so maybe portion creep (if a peron is calorie counting/logging carefully, they'd likely know if this is happening; if just "watching what one eats" or logging with some estimating and approximating in the mix, this also can be a pretty subtle effect).

Bottom line: CICO - the calorie balance formula - still applies. Get calories below calories burned, weight loss happens. With medication, calorie counting may experience some bumps in the road, if calories burned drop due to fatigue, or calories eaten increase due to portion creep. (A few meds could even cause peaks and valleys in fatigue or eating, make counting even more confusing, even those CICO still applies! But that's rare.)

If the issue is water retention, fat loss is still happening, just being masked on the scale by the extra water weight. Some (unusual) things can cause creeping increases in water retention, up to a surprising total amount. It's more common for water retention just to move a person from X pounds of water, to X plus a couple or so pounds of water, then stop increasing. In that common case, fat loss will eventually outpace that couple of pounds or so, and show up on the body weight scale.

Clearly, the person involved should be consulting with the doctor who prescribed the meds, too, to ask about alternatives with fewer or different side effects (or treatments for them), or to ask for referrals to a registered dietitian for help with eating in their individual health context and maybe even a physical therapist for appropriate activity ajdustments (though it sounds like your friend is OK there) . Bodies are pretty subtle, complicated things - the pros can help us sort things out, usually.