Feeling awful 4.5 months in on Mounjaro 5mg

[Toolea]

I’ve been on MJ since Jan 2026. Have lost nearly 2.5 stone and am now about 10lbs away from target weight and am currently in top end of normal BMI so would like to go down a bit more.
Was on 2.5mg for ages and then went up via a few weeks on 3.75 to 5mg as wasn’t getting suppression anymore. Took my 4th dose of 5mg this Monday past and had the worst headache on Wednesday. Have been very fatigued and a bit dizzy over the months but feeling even more so now plus low/blank mood and my chest feeling a little raw at times (I saw GP recently so not concerned about this as being anything other than MJ). But have generally been too exhausted to exercise whilst on MJ which is not great.
So annoying because the jab has been great for my immune conditions and I now also don’t need my CPAP machine anymore. I’d hoped to stay on 5mg long term as suppression is good but the way I feel now I just feel too unwell and exhausted. I’m flying off abroad for a week’s holiday on Monday (when my 5th dose of 5mg is due) and I’m wondering about just taking 2.5mg to try and alleviate these symptoms while I’m away - worried about ruining our holiday tbh. Obviously also worried about eating like a horse though!
Chat gpt says that week 4 on 5mg can lead to all these issues as it’s the highest the MJ has ever been in my system and is a well known effect? Is that right?
Has anyone else had this happen to them?

The half life of mounjaro is 5 days. That means after 5 days half the drug is still circulating, then after another 5 days half THAT, and so on. The amount halves every five days.

This means that if you keep taking more every seven days, over the month the amount in your system will rise, as on week 4 you have the last gasp of week 1, then twice that of week 2, then twice that of week 3, plus week 4.

This is true of all drugs not just MJ, though the half life differs. For example for paracetamol it's more like 3 hours, not five days.

I am not a doctor and this is not medical advice, but I can tell you that personally I am taking much lower doses because I really felt very fatigued on 5mg and I have had much better results going lower. I have a job, you know, I can't be falling asleep at 2pm. I have settled on about 1mg and I'm still getting the reduced inflammation (eg normal CRP etc) benefits. I am not losing weight at the speed I was, more like 1 pound a month! But I'm happy with my normal BMI and normal levels of inflammation, for the first time in many years.

Headaches and dizziness on mounjaro can be caused by dehydration as well. You can try diarolyte. I get vertigo on mounjaro sometimes and drinking electrolytes fixes that.

How much are you eating? You say suppression is good, are you eating enough? Lack of energy could be due to calories being too low

Get your iron levels checked - they are probably too low

Around 1,200 which at 5 foot 4 is what I need to lose about a lb a week. I’ve actually stalled for the past few weeks so feeling so rubbish is even more annoying.

Thank you. Good idea!

That’s very helpful, thank you. I’ll get some today.

Thanks so much for sharing your experience and explaining the half life. It’s very helpful and reassuring. Think I will definitely go back down to 2.5mg on Monday as if I take another 5mg I can imagine I’ll feel pretty bad on my hols. Will reassess the situation when I get back.

Maybe increase the time between doses & I’d up food intake for few days. Sounds counter productive but it’s not. Your body is in starvation mode so your metabolism has dropped - you need to fire it up, esp if not exercising.

Good advice. I suspect my food intake will be radically upped when I’m on holiday 😆

If it helps I’m finding that drinking 2-3 litres of water really does stop the headaches - I’m on week 14. Also I’m creeping up at .25 increments and ONLY when I really need to, currently on 2.75 mg. I also now take electrolytes twice a day and eat absolutely no UPFs.

If it helps I’m finding that drinking 2-3 litres of water really does stop the headaches - I’m on week 14. Also I’m creeping up at .25 increments and ONLY when I really need to, currently on 2.75 mg. I also now take electrolytes twice a day and eat absolutely no UPFs.

If it helps I’m finding that drinking 2-3 litres of water really does stop the headaches - I’m on week 14. Also I’m creeping up at .25 increments and ONLY when I really need to, currently on 2.75 mg. I also now take electrolytes twice a day and eat absolutely no UPFs.

If it helps I’m finding that drinking 2-3 litres of water really does stop the headaches - I’m on week 14. Also I’m creeping up at .25 increments and ONLY when I really need to, currently on 2.75 mg. I also now take electrolytes twice a day and eat absolutely no UPFs.

Lots of electrolytes and water.

However I think you’d be better dropping to 2.5 or 3.25 and lifting weights. You say you’re not exercising so therefore your body will be feeling really weak.

Thank you ❤️

Yes, I feel very out of condition. Did a little cardio and light weights a few days ago and felt really dizzy and shattered. Thank you for the good advice.

I found 5mg a little high for me. Dropped back to 4ish and then had side effects similar to yours for a couple of weeks despite being on it for 9 months with no issues.

I then went on holiday. Jabbed 4ish a little early (didn't want the runs on the plane!), and skipped a dose as couldn't be bothered to take my pen with me. Ate a lot of nice food (went to Florida's) walked 25k steps per day and jabbed 4 when I got home and had no other side effects! All back on track. The moved to 5mg with no issue.

Tried 6.2 recently and had exactly the same so now back at 5mg again.

try a little lower and a small break. Changing diet also helps as a reset.

I also had lots of plateaus as well but overall still creeping down.

This is super helpful, thank you. Can I ask, did you have this sort of a raw feeling in the chest occasionally? And was your mood lowered? I feel like a knackered zombie right now tbh.

@Toolea I wouldn't say mood lowered as such, but definitely tired, and more lethargic. The stomach issues made me feel grim, but the headaches impacted more. The break absolutely gave me a reset, but as everyone says don't forget to check iron too as that can be similar.

I've been on MJ for 18months now and had a couple of blips but it normally sorts itself out (and as I say I'm back at 5mg now). I also had an epic stall before Florida, like 9 weeks of no losses. And now back on it no issues (Florida was October), although very very slow and steady downward creep rather than huge losses.

Thank you. It’s so helpful to hear others experiences and I’m really grateful to you for taking the time. Slow and steady wins the race…

If it helps I’m finding that drinking 2-3 litres of water really does stop the headaches - I’m on week 14. Also I’m creeping up at .25 increments and ONLY when I really need to, currently on 2.75 mg. I also now take electrolytes twice a day and eat absolutely no UPFs.

That sounds really rough 😕 I had a few weeks on Mounjaro where the fatigue hit me properly after going up a dose and it honestly knocked me abit. I’m with an online provider and the weekly app check ins plus clinical oversight helped me feel less like I was guessing what to do. The simple nutrition tips and recipes helped too when I couldnt face cooking much. Hope you manage to enjoy your holiday atleast.