Weight-Based Dosing: What to Know About Medication Dosages and Body Weight

If you’ve ever purchased medication over the counter (OTC) from your local pharmacy or retailer, you might’ve noticed different dosages and directions listed on the label. There may have even been a dosing chart displayed on the box. These recommendations are often based on body weight, a method called weight-based dosing.

Many prescription-only medications also use weight-based dosing, but you may not have realized it because your provider determines your dose for you. 

So, you may be wondering: Why are some medication dosages based on body weight? And how does weight-based dosing even work? We’re here to answer all of your questions.

Why are some medication dosages weight-based?

Some medication dosages are weight-based because your body weight or body composition could affect their absorption, distribution, metabolism, or elimination. These four factors predict how a medication will behave in your body. Drug toxicity can also play a role.

Changing any one of these factors can potentially make a medication more or less active in your body. In turn, this can lead to unwanted side effects or a less effective treatment.

If body weight or body composition impacts a medication’s safety or effectiveness in initial studies, the medication’s manufacturer may recommend weight-based dosing.

Examples of weight-based medications for adults

Several medications for adults are dosed based on body weight. These are a few examples:

• Insulin

• Enoxaparin (Lovenox)

• Levothyroxine (Synthroid, Levoxyl, Unithroid)

• Antibiotics like vancomycin

• Anesthetics like propofol (Diprivan)

Examples of weight-based medications for kids

Many medications for kids are dosed based on body weight (and age). It’s a long list, but these are some common examples:

• Acetaminophen (Children’s Tylenol, Infants’ Tylenol)

• Ibuprofen (Children’s Motrin, Children’s Advil)

• Diphenhydramine (Benadryl)

• Prednisone

• Tamiflu (oseltamivir)

• Antibiotics like amoxicillin (Amoxil)

How does weight-based dosing work?

Weight-based dosing uses your body weight to determine a safe and effective medication dose for you.

The first step to figuring out your dose is to take an accurate measurement of your current weight. If you’re at your healthcare provider’s office, a staff member will likely help you with this. If you’re at home, you can follow these seven steps to measure you weight or your child’s weight:

1. Take your measurement in the morning before you eat.

2. Use a digital scale, if possible. It’s best to avoid spring-loaded bathroom scales for accuracy’s sake.

3. Place the scale on a hard, firm surface, such as wood or tile flooring. Avoid placing the scale on carpet.

4. Make sure the scale is properly calibrated.

5. Remove your shoes and any heavy clothing items. It’s also a good idea to empty your pockets.

6. Step onto the scale, keeping both feet centered.

7. Record the weight displayed on the screen. Pay attention to the units at the end of the number, which will most likely be pounds (lbs) or kilograms (kg).

Once you have your weight, you or your healthcare provider can figure out your dose. If you’re using a chart on an OTC product, find your weight — or the range that includes your weight — and then find the corresponding dose. Again, pay attention to units. If the scale gives you your weight in pounds, but the chart uses kilograms, you’ll have to convert pounds into kilograms, or vice versa. Keep this conversion handy: 1 kg equals 2.2 lbs.

For prescription medications, it’s common for weight-based dosages to be even more personalized. They’re usually recommended in terms of the amount of medication per one unit of body weight.

Your provider or pharmacist will most likely do these calculations for you, but let’s discuss an example: Let’s say you weigh 60 kg, and the recommended dose for your medication is 2 mg per kg of body weight. This means that you must take 2 mg of medication for every kg you weigh. In this case, your weight-based dose would be 120 mg.

Other types of weight-based dosing

Some medication dosages are calculated using different measurements based on body weight, such as “ideal body weight” or “adjusted body weight.” These measurements take additional components of your body’s makeup into account to provide an even more accurate dose based on the properties of the drug.

Other medications, such as chemotherapy medications like carboplatin, may be dosed based on your body surface area (BSA). BSA uses a strategy that takes both your weight and height into account.

You most likely won’t have to worry about these calculations. You won’t find these medications OTC, nor will your healthcare provider ask you to determine these measurements yourself.

Are all medications dosed according to body weight?

No. Not all medications are dosed according to body weight. 

In fact, you’re probably most familiar with a method called “fixed dosing.” Fixed dosing means that everyone of a particular age group gets the same dose for their respective health condition. Factors like body weight have little-to-no effect on your medication’s safety or effectiveness.

Most medications use fixed dosing. For instance, a typical starting dose of lisinopril (Zestril) for many adults with high blood pressure is 10 mg once daily.

Even though body weight doesn’t usually play a role in fixed dosing, other factors can still make a difference. Considerations like kidney or liver health, age, and sex can make your dose higher or lower. Drug interactions can also play a role.

What are some challenges or drawbacks of weight-based medication dosing?

Weight-based dosing helps promote medication safety, but it does come with some challenges.

Convenience is the main drawback. Compared to fixed dosing, weight-based dosing can be less convenient for healthcare providers, caregivers, and people who take medications. There are multiple steps needed, such as measuring your weight and performing calculations. Wouldn’t it be easier for everyone if we could all take the same dose?

Using this same thinking, another possible drawback of weight-based dosing is medication errors. The more steps involved in determining the dose, the higher the chance of mistakes.

Another challenge is body weight fluctuations. If you have significant changes in your weight but your provider isn’t aware of them, your medication dose may no longer be appropriate. So, if you experience any big changes in your body weight, make sure to keep your provider in the loop.

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Written by:

Jenna Clayton, PharmD

Jenna Clayton, PharmD earned her Doctor of Pharmacy degree at the University at Buffalo School of Pharmacy and Pharmaceutical Sciences in Buffalo, New York. Throughout her doctoral program, Jenna gained experience in a variety of pharmaceutical settings, including community, hospital, long-term care, and ambulatory care settings.

Edited by:

Joshua Murdock, PharmD, BCBBS

Joshua Murdock, PharmD, BCBBS, is a licensed pharmacist in Arizona, Colorado, and Rhode Island. He has worked in the pharmacy industry for more than 10 years and currently serves as a pharmacy editor for GoodRx.

Reviewed by:

Stacia Woodcock, PharmD

Stacia Woodcock, PharmD, is a pharmacy editor for GoodRx. She earned her Doctor of Pharmacy degree from the University of Kentucky and is licensed in New York and Massachusetts.

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