Eating too little on a weight loss jab: the warning signs

SASian Anderson, Writer and editor, MyGLP. Sources checked 21 October 2026.

How it happens

The medicines are good at their job. Some people go from eating too much to eating almost nothing, and because they feel fine for a while, nobody notices. A pharmacy prescriber sees weight going down and is pleased. The person is pleased. The body is not.

The NHS treats diets under 800 calories a day as very low calorie diets, to be followed only for up to 12 weeks and only with medical supervision. Plenty of people on a weight loss medicine drop below that line by accident. There is no trial evidence that eating that little on the medicine gives faster fat loss. There is evidence that it gives more muscle loss, more hair loss and more gallstones.

The signs

• Feeling cold when others are not.
• Dizziness when you stand up.
• Tiredness that sleep does not fix.
• Hair shedding, three to four months in. See the hair loss guide.
• Constipation despite fibre and fluid, because there is nothing to move.
• Periods going light or stopping. See the periods guide.
• Low mood, irritability, poor concentration.
• Muscle loss you can see: looser arms, weaker grip, stairs harder.
• Losing more than 1kg a week, every week, after the first month.

Gallstones

Gallstones are listed as a common side effect in the Wegovy leaflet and on the NHS tirzepatide page. The link is with fast weight loss by any method: the gallbladder empties less when you eat little fat, bile sits, and stones form. Women are more prone to them than men. The NHS gallstones page describes the pain: a severe ache under the right ribs or in the centre of the belly, often after a meal, lasting from minutes to hours. Pain like that needs a GP appointment, and pain with fever or yellow skin needs urgent help.

Slower weight loss and regular meals, even small ones, with a little fat in them, are the two things that lower the risk.

A floor, not a target

There is no single right calorie figure. As a working floor on a weight loss medicine, most dietitians would want to see at least 1,000 to 1,200 calories a day for a woman, with protein at 1.2g per kg or more, unless a doctor is supervising something lower. If you are nowhere near that, the aim is to come up, not to eat less.

How to eat enough without forcing it

1. Eat by the clock, not by hunger. Hunger is switched off. Set three meal times and two snack times and eat something at each, no matter how small.
2. Protein first at every one. A boiled egg, a pot of yoghurt, a slice of chicken. The protein guide has a day that hits 100g in six small servings.
3. Add calories that take no room. Olive oil on vegetables, nut butter on a cracker, full-fat yoghurt instead of 0%, a handful of nuts. A tablespoon of olive oil is 120 calories and a sip of effort.
4. Drink some of it. Milk, a smoothie with protein powder, or a soup gets food in when chewing feels like work.
5. Track for a week. Not forever. One week in a free app shows you whether you are at 700 calories or 1,300, and most people guess wrong.
6. Ask to hold the dose. If you cannot get above the floor, the prescriber can keep you at the current step for longer rather than moving up. The weight still comes off.

When the medicine is doing too much

If you have no appetite at all, cannot eat a small meal without feeling sick, and the signs above are stacking up, that is a reason to speak to the prescriber, not something to push through. Dropping back a step is a normal adjustment. The NHS tirzepatide page says to speak to a pharmacist or doctor if side effects bother you or do not go away, and under-eating is a side effect by another name.

Sources

1. Obesity: treatment, including very low calorie diets (NHS)
2. Gallstones (NHS)
3. Tirzepatide: side effects (NHS)
4. Wegovy patient information leaflet (Novo Nordisk, via the electronic medicines compendium)
5. Beat, the UK eating disorder charity (Beat)