Weight loss surgery and medicines

Weight loss surgery is also called bariatric surgery

Weight loss surgery can change the way your body absorbs medicines which will affect the dose or type of medicine you need to take. Learn more about weight loss surgery and medicines.

How does weight loss surgery affect medicines?

There are a number of different weight loss surgeries. Some, like sleeve gastrectomy, reduce the size of your stomach and therefore the amount of contents in the stomach at one time. Other surgeries like Roux-en-Y gastric bypass, skips some areas of the gut. These and other effects of weight loss surgery can affect the way your body absorbs medicines. Your healthcare provider will advise you on the changes needed to your medicines based on the type of surgery you have and changes to your weight.

Here are some general tips on managing medicines after weight loss surgery.

Get your medicines checked and reviewed regularly

The effect of weight loss itself after bariatric surgery may require medicine doses to be altered.

  • People who are obese often have other conditions such as high blood pressure, diabetes and pain from osteoarthritis.
  • These conditions may improve with weight loss so you may need a change of dose of some of your medicines.
  • Also, medicines where the dose in based on weight, would need to be adjusted as you lose weight.

Weight loss surgery can affect the way some medicines are absorbed in the gut.

  • Some medicines may be better absorbed which may mean you need a lower dose to get the same effect or some medicines may be  poorly absorbed which may require a higher dose for the same effect.
  • This is especially important for medicines where a small change may lead to serious side effects such as lithium, digoxin, medicines for epilepsy and anticoagulants. 

If you are unsure if you need to change your dose, or change to another type of medicine, talk to your healthcare provider.

Some medicine formulations should be used with caution or avoided

Large capsules and tablets

Larger capsules and tablets (over 10 mm) can become stuck, especially in the two weeks after surgery.  Check with your pharmacist if your tablets can be crushed, or taken in a liquid form. Some types of medicines are also available as injections or implants.

Enteric-coated formulations

The acid in the stomach may be altered after certain weight-loss surgery. Enteric-coated medicines have a special coating that may need the acidic environment of the stomach to dissolve the coating before the medicine can be absorbed in the intestine. If you are taking enteric-coated medicines, these may need to be changed, depending on the type of surgery.

Sustained or delayed release formulations

These often require longer passage through the gut to be effective. Reducing the size of the gut can affect how well they work.  

Effervescent formulations 

Effervescent tablets or powders create a fizz and dissolve when they are added to water. The fizz causes the release of gas (carbon dioxide). These medicines should be avoided after weight loss surgery because of the build-up of gas can become trapped in the stomach and cause discomfort and pain. 

Liquid formulations

Because of the reduced size of the stomach after some types of surgery, liquid medicines may need to be taken in smaller doses more often. You may be given a higher concentration of your liquid medicine to reduce the volume of each dose. 

Be aware of dumping syndrome

After gastric bypass, avoid products (including over-the-counter products) that contain a large amount of sucrose, corn syrup, lactose, maltose, fructose, honey or mannitol, as they can result in dumping syndrome. Dumping syndrome happens when the contents moves down from your stomach into your small bowel too quickly after you eat. This can cause tummy cramps and diarrhoea (runny poo) after eating. 


  1. Bariatric surgery and medicines: from first principles to practice Australian Prescriber, October 2022
  2. Gastrointestinal surgery – Consider possible effects on medicine pharmacokinetics Medsafe NZ, September, 2022
  3. Bariatric surgery patients and their medicines NHS 2014
Credits: Sandra Ponen, Pharmacist. Reviewed By: Angela Lambie, Pharmacist, Auckland Last reviewed: 26 Oct 2022