【Know This Before Your First Shot】Ozempic or Fasting, Which Is Better? A Thorough Comparison of GLP-1 Drug Effects, Side Effects, Costs, and Weight Regain

“I keep hearing about Ozempic lately. Does it really work?”

“How does it compare with fasting? Which one is better, and how much does it cost?”

GLP-1 receptor agonists (such as Ozempic and Wegovy) are weight-loss drugs that have drawn worldwide attention over the past few years.

In Japan, too, the number of people asking for a prescription has been rising fast.

Meanwhile, research has been showing that fasting can also curb appetite through a mechanism similar to GLP-1.

In this article, I’ll compare GLP-1 drugs (Ozempic) and fasting in depth across four areas, effectiveness, safety, cost, and sustainability, based on the data.

Use it as a guide to figure out which one suits you better.

What Are GLP-1 Drugs (Ozempic)? How They Work, Explained Simply

What Is GLP-1 (Glucagon-Like Peptide-1)?

GLP-1 is a hormone released from the small intestine when you eat.

It has three main roles.

Ozempic (generic name: semaglutide) is a drug that mimics the action of GLP-1, and it has been shown to strongly suppress appetite and reduce body weight with a once-weekly self-injection.

An Analogy for How GLP-1 Drugs Curb Appetite

GLP-1 is like a messenger that tells the brain, “You’ve eaten enough,” on behalf of the gut.

The GLP-1 your body makes naturally gets broken down after a while, and its job is done.

It’s easiest to think of GLP-1 drugs as being designed so that this messenger can keep working for much longer.

Because the “I’m full” message keeps reaching the brain, you feel satisfied more easily even with smaller portions.

On top of that, the exit of the stomach opens more slowly, so food stays in the stomach longer.

Ozempic’s Effects: What the Clinical Data Show

In a large clinical trial published in The New England Journal of Medicine (NEJM) in 2021,
the group given 2.4 mg of semaglutide (Wegovy, the version for obesity) once a week for 68 weeks achieved an average weight loss of about 14.9% (compared with 2.4% in the placebo group).

That’s a very high figure for a weight-loss drug, and it’s one of the reasons it attracted so much attention.

Note that in this trial, the group taking the drug also received diet and exercise counseling.

They didn’t lose weight from the drug alone; it was paired with lifestyle changes, and that point shouldn’t be overlooked.

How It’s Obtained and Prescribed in Japan

In Japan, Ozempic is approved as a treatment for type 2 diabetes, while Wegovy (the same semaglutide at a higher dose) was approved for treating obesity in 2023.

However, insurance coverage comes with conditions, and more and more clinics are prescribing it as self-pay (uninsured) treatment.

What to Know Before Using It for Weight Loss

For people without diabetes, using Ozempic to lose weight is “off-label use,” meaning it differs from the approved use.

As cosmetic use has spread, concerns have been raised that the drug may become harder to obtain for diabetes patients who truly need it, and groups such as the Japan Diabetes Society have issued warnings.

Also keep in mind that if serious side effects occur from off-label use, you may not be covered by Japan’s national “Relief System for Sufferers from Adverse Drug Reactions.”

When you’re deciding whether to use it, tell your doctor accurately about your health, hear their explanation of the benefits and risks, and then make your decision.

Does Fasting Also Increase GLP-1? The Science Behind It

GLP-1 Rises in the Body During Fasting

This is one of the reasons why “even a small amount makes you feel full” at the meal after a fast.

If you feel full more easily at the meal after fasting, overeating will naturally decrease.

In other words, the difference is that GLP-1 drugs supply GLP-1 from the outside, while fasting draws out GLP-1 secretion from inside the body.

For more on how the body changes during fasting, see how ketone bodies burn fat during a 16-hour fast.

Another Reason Fasting Calms Your Appetite

GLP-1 isn’t the only hormone involved in appetite.

“Ghrelin,” released from the stomach, is a hunger hormone that tells the brain you’re hungry.

Ghrelin is thought to tend to rise around the times you usually eat.

If you graze all day long, your body stays on alert all day, thinking, “It’s almost mealtime.”

When you set your eating times with fasting, the times when hunger waves arrive also start to line up to some extent.

It’s similar to how you can wait calmly on the platform when you know the train will arrive right on schedule.

Weight Loss From Fasting: What Does the Research Say?

A systematic review published in the journal “Canadian Family Physician” in 2020
looked at 27 trials of intermittent fasting (944 people in total, most lasting 2 to 26 weeks) and concluded that body weight decreased by 0.8% to 13.0% of baseline weight, and no serious adverse events were reported.

Compared with the roughly 14.9% for GLP-1 drugs, these numbers are a little lower, but fasting differs greatly in that you can keep it up without side effects, at zero cost, and at your own pace.

For more on fasting and the latest medical research, also read the health benefits of fasting proven by medical journals such as NEJM.

When I learned that fasting increases GLP-1, I thought, “So I can do the same thing with my body’s own mechanisms, without using a drug.”

Of course the drug is more powerful, but I think being able to draw out GLP-1 naturally is a major strength of fasting.

GLP-1 Drugs vs. Fasting: An In-Depth Comparison Across 4 Areas

1. Effectiveness: Comparing Weight-Loss Power

Looking purely at the numbers for weight loss, GLP-1 drugs (Ozempic) show a stronger effect than fasting.

That said, fasting’s effects can be boosted by adjusting the duration and method.

Some researchers say that when continued over the long term, fasting offers broader overall health improvements.

It’s important to consider not just how much weight you lose, but also how long you can keep it up and what happens after you stop.

2. Safety: Differences in Side Effects and Risks

The main side effects reported for GLP-1 drugs are as follows.

With 16-hour fasting, serious side effects have rarely been reported in healthy adults.

Some people feel hunger, mild headaches, or fatigue in the first few days, but most get used to it within one to two weeks.

However, if you’re pregnant, breastfeeding, or have certain medical conditions, please consult your doctor before trying it.

3. Cost: Comparing Expenses Over the Long Term

When it comes to cost, the gap between the two is huge.

One challenge with GLP-1 drugs is that they cost money for as long as you keep taking them, and your weight tends to come back when you stop.

Once fasting becomes a habit, you can maintain it for life at almost no cost.

For example, taking a ¥50,000-a-month drug for a year adds up to ¥600,000, enough to pay for a family trip or a new home appliance.

4. Sustainability: Which Can You Keep Up Longer?

GLP-1 drugs require a weekly self-injection and come with dealing with side effects, regular doctor visits, and ongoing costs.

Fasting needs no special tools, and its strength is that you can keep it up the same way while traveling or on busy days.

With an injectable drug, you’ll also need to handle daily management, such as how to store it and how to dispose of used needles.

Fasting is hardest in the first two to three weeks while you adjust, but once you’ve found your rhythm, it becomes “part of your life” that you can continue almost without thinking about it.

For tips on making fasting a habit, also see tips for keeping up fasting for 21 days.

Which Should You Choose? A Decision Guide by Type

Who GLP-1 Drugs Are Suited For

Even in these cases, combining fasting and better eating habits alongside the GLP-1 drug makes it easier to prevent weight regain after you stop the medication.

Who Fasting Is Suited For

To learn how to take your first step with fasting, start with the complete guide to intermittent fasting.

If You Use a GLP-1 Drug, Think Ahead to “After You Stop”

If you use a GLP-1 drug, the key is to plan from the start for how you’ll live after you stop taking it.

While the drug is working, your appetite is naturally held in check, so eating less doesn’t feel like a struggle.

But when you stop the drug, it’s as if those “training wheels” come off.

If you don’t practice pedaling while the training wheels are on, you’ll probably fall over the moment they come off.

The key to preventing weight regain is to gradually build habits while you’re still on the drug, such as setting your eating times, getting enough protein, and moving your body.

How to Think It Through When You’re Unsure

If you can’t decide which to choose, first check whether you’re in a condition that medically requires the drug.

If your health checkup has flagged your blood sugar or weight, don’t decide on your own; the quickest route is to talk to your regular doctor.

If you’re worried about future illness, for example because someone in your family has diabetes, it’s reassuring to consult a doctor once as well.

If you don’t particularly need treatment, it’s not too late to start by improving your lifestyle habits with fasting.

Whichever you choose, writing down “why I want to lose weight” makes it easier to see which method suits you.

Precautions to Avoid Mistakes With Ozempic and Fasting

Be Careful Combining Diabetes Medication and Fasting

If people taking blood-sugar-lowering medication fast on their own judgment, there’s a risk of “hypoglycemia,” where blood sugar drops too low.

Hypoglycemia can cause cold sweats, trembling hands, a pounding heart, and intense hunger, and in severe cases you may even lose consciousness.

If you’re on medication, be sure to consult your doctor and have them adjust the dose and timing of your medication before you start.

Avoid Importing It Yourself Over the Internet

Personally importing GLP-1 drugs from overseas or buying them from unofficial online shops is extremely dangerous.

Regulators overseas have also issued warnings that counterfeit products of unknown content are in circulation.

If you use the drug, always get it prescribed after being examined by a doctor at a medical institution.

Whichever You Choose, Eat in a Way That Protects Your Muscles

When you lose weight, you tend to lose not only fat but also muscle.

Muscle is like the engine that burns energy in your body.

When the engine gets smaller, your body gains weight more easily even if you eat the same amount.

Make a point of including protein such as meat, fish, eggs, and soy products at every meal, and combine it with walking or light strength training.

Don’t Miss the Signals Your Body Sends

If you’re using a GLP-1 drug and have persistent severe abdominal pain or nausea, don’t put up with it; contact the medical institution that prescribed it right away.

In particular, severe abdominal pain that radiates to your back can be a sign of pancreatitis, a rare side effect.

With fasting, too, it’s important not to dismiss changes in how you feel as “just something to endure until I get used to it.”

Your body’s signals are like the warning lights that come on in a car’s dashboard.

Keeping simple notes on changes in your weight, waist, appetite, and how you feel will also help when you talk to a doctor.

Being able to see small changes with your own eyes also helps keep you motivated.

Who Should Hold Off on Fasting

If any of the following apply to you, don’t start fasting on your own judgment; consult a doctor.

If you feel dizzy, extremely tired, or break out in a cold sweat while fasting, don’t push yourself; eat something right away.

Frequently Asked Questions About Ozempic (GLP-1 Drugs) and Fasting

Fasting, Which Works With Your Body’s Own Mechanisms Instead of Drugs, Is the Smartest Long-Term Choice

GLP-1 drugs (Ozempic) are certainly powerful weight-loss drugs.

However, considering the cost of ¥30,000 to ¥80,000 a month, the risk of weight regain when you stop, and the issue of side effects, they’re not an option that can be recommended to everyone.

Fasting, on the other hand, makes use of your body’s natural GLP-1 secretion and is a way to build a body that resists weight regain, without side effects and at almost no cost.

If you look only at speed and numbers, GLP-1 drugs come out ahead, but
from the standpoint of maintaining a healthy body shape over 10 or 20 years, fasting is overwhelmingly the more rational choice.

What matters is not to get too fixated on a simple either-or choice between drugs and fasting.

People who medically need treatment should use the drug while consulting their doctor, and everyone else can start with fasting, which makes use of the body’s own mechanisms.

Choosing based on your own body’s condition may look like a detour, but it’s actually the shortest path.

Start with a 16-hour fast.

Even just finishing tonight’s dinner a little earlier is a first step toward putting your body’s mechanisms to work.

Once your body starts to change, you’ll gain the confidence that you can control your body on your own, without relying on drugs.

If you’d like to make fasting more comfortable, another option is to use a supplement that supports fasting.

It helps ease hunger while supporting your natural GLP-1 function.

▼ Watch the video version of this article