Month three on GLP-1, I was down 14 pounds and should have felt good. Instead I was tired in a way that felt different from regular tiredness. Not sleepy. More like the tank was simply lower than it used to be.
I was also trying to maintain my old exercise routine, which wasn’t working. A 45-minute workout left me useless for the rest of the day. I’d rest, try again, and the same thing would happen.
After talking to my provider and doing a lot of reading, I understood what was actually going on. And more importantly, I found what to do about it.
Why GLP-1 Causes Fatigue
There are a few things happening at once.
You’re eating significantly less. GLP-1 medications suppress appetite effectively. Most people reduce their caloric intake substantially without trying. Fewer calories means less fuel available for physical activity. This is the most direct cause of exercise-related fatigue on GLP-1.
Your body is adjusting to the medication. GLP-1 affects digestion, blood sugar regulation, and energy metabolism. In the first two to four months on a new dose, the body is still adapting. Fatigue during this window is normal and typically improves as the adaptation completes.
Muscle loss contributes over time. If you’re losing muscle along with fat — which is common on GLP-1 without resistance training — your overall capacity for physical output decreases. Less muscle means less energy-producing tissue. This creates a compounding problem if it isn’t addressed.
Cellular energy production can decline with age. This is separate from GLP-1 but relevant for people over 40. The mitochondria in your cells — the structures responsible for producing usable energy — become less efficient with age. GLP-1 on top of that doesn’t create the problem, but it can make it more noticeable.
What to Do When You’re Too Tired to Work Out
Reduce the session, don’t skip it
The worst response to fatigue is all-or-nothing thinking. If you’re too tired for a 40-minute strength session, do 15 minutes. Three exercises, two rounds, done. Getting the stimulus is more important than hitting a duration target.
Walking counts. On the days when any structured exercise feels impossible, a 20-minute walk after dinner keeps the habit intact and provides real metabolic benefit. It is not a failure to walk instead of lift. It’s a smart adjustment.
Check your protein intake
When appetite is suppressed, protein is often the first macronutrient to fall short. Low protein accelerates muscle loss, which worsens fatigue over time. It also impairs recovery from any exercise you do manage.
Aim for a minimum of 80 grams of protein per day on GLP-1, with 100 to 120 as the better target for anyone doing resistance training. A protein shake is a practical way to hit that number on low-appetite days.
Don’t train in the evening if you’re already depleted
Morning or midday training tends to work better on GLP-1 than evening sessions, because you haven’t spent the whole day drawing down your energy reserves. If you’re consistently hitting the wall in afternoon or evening workouts, try shifting the session earlier.
Look at sleep quality
GLP-1 changes digestion and eating patterns, which can affect sleep in the early months. If you’re waking frequently or not sleeping deeply, that’s compounding the fatigue problem. Sleep is when muscle repair happens and when the body restores energy reserves.
When Fatigue Points to Something Else
Persistent, significant fatigue that doesn’t improve after the first two to three months warrants a conversation with your provider. Several things worth ruling out: low iron, vitamin D deficiency, thyroid function, and overall caloric intake being too low even by GLP-1 standards.
Fatigue that’s specifically an afternoon energy crash — present even on rest days, not just after exercise — may be related to cellular energy production rather than exercise load.
NAD+ and Energy on GLP-1
I started NAD+ therapy in month five on GLP-1. I was sleeping adequately, eating reasonably well, and still hitting an afternoon wall that felt disproportionate.
NAD+ is a coenzyme involved in cellular energy production. Levels decline with age, particularly after 40, which is part of why energy management becomes harder as we get older. NAD+ therapy — delivered through injection or IV — can support mitochondrial function and improve energy levels in people with depleted NAD+ reserves.
By week three of NAD+ therapy, the afternoon crash was noticeably less severe. By week six, I was completing workouts that had felt impossible two months earlier. I can’t attribute that entirely to NAD+, but the correlation was clear enough that I’ve continued it consistently.
The program I use is through ShedRX. They offer NAD+ therapy as part of their telehealth platform, the same provider I use for GLP-1. Details on the NAD+ program are here.
Affiliate link — I may earn a commission at no extra cost to you.
The Realistic Timeline
Months one and two on a new GLP-1 dose are typically the hardest for energy. Nausea, adjustment to reduced food intake, and the body’s adaptation to the medication all compete for your resources.
By months three and four, most people find a level of energy that, while lower than their pre-GLP-1 baseline, is workable. By month six, with strength training established and protein intake stable, energy during exercise typically improves meaningfully.
The goal in the early months isn’t to maintain your previous fitness level. It’s to keep moving at whatever level you can manage, protect your muscle, and let the adaptation run its course.