Factors to consider when using medicines following bariatric surgery (gastric bypass)

Paula Russell, Principal Pharmacist, Regional Drug and Therapeutics Centre (Newcastle Upon Tyne)Published Last updated See all updates

Guidance on the physiological changes after bariatric surgery that can affect the absorption of medicines and supplements.

The importance of considering medicines and bariatric surgery

Bariatric surgery is recommended by NICE Clinical Guideline as a treatment option for obese patients who meet specific criteria.

In England in 2017, 27% of adults in the general population were obese and about 35% of the population are projected to be obese in 2030. Given that obesity is rising, it is likely that the number of patients eligible for bariatric surgery will also increase so prescribers understanding of the likely effect medicines is increasingly important.

Factors that you should consider when prescribing for a patient after bariatric surgery include the type of surgery, drug considerations such as altered pharmacokinetics, and patient choice.

Stomach size


The stomach is the primary area for disintegration of standard release tablets or capsules. This process involves a combination of mechanical forces, gastric acid, and saliva.


Restriction in the size of the stomach pouch could therefore lead to a less complete disintegration.

pH changes


The pH within the gastric pouch created by bariatric surgery is higher than that found in a normal stomach.


This may reduce the absorption of medicines which are soluble in acidic environments. Those that are soluble in alkaline environments are mainly absorbed in the small intestine.

In restrictive malabsorption procedures, the majority of the small intestine may be bypassed, reducing the amount of drugs which are absorbed in this area.

Surface Area


Restrictive-malabsorption procedures significantly reduce the amount of surface area in the GI tract available for drug administration.


Drugs which usually remain in the intestine for longer whilst being absorbed are likely to have their bioavailability reduced.


Drugs which are primarily absorbed in the duodenum and jejunum may need to be given via a different route where possible, or doses may have to be adjusted accordingly.



There is a high prevalence of nutritional deficiencies in both adults and adolescents with severe and complex obesity. Bariatric surgery impacts both oral intake and absorption increasing the risk of nutritional deficiencies.


Restrictive procedures are less likely to cause nutrient deficiencies than combination procedures.

Patients may be prone to deficiencies of the fat soluble vitamins (A, D, E & K), calcium, iron, vitamin B12, and folate.


Following all bariatric procedures, a complete multivitamin and mineral supplement (containing thiamine, iron, zinc, copper and selenium) is recommended.

Following Sleeve Gastrectomy (SG), Roux-en-Y gastric bypass (RYGB) and malabsorptive procedures such as Duodenal Switch (DS), additional elemental iron and vitamin B12 may be required.

Patients who develop anaemia should be treated symptomatically. Vitamin B12 IM should be given 3 monthly.

Drug Formulation


The ability to absorb drugs is changed by bariatric surgery. The openings produced by certain types of bariatric surgery may be only 1-1.5cm wide.


Drugs administered in an aqueous solution are more rapidly absorbed than those in solid form or oily solutions.


Release characteristics

Immediate release formulations are preferred. Extended release formulations should be avoided. Enteric or film coated tablets may also be problematic, as any delay in drug release may reduce absorption.

Effervescent formulations

Some centres advise avoiding carbonated beverages after the surgery, as they often cause discomfort. Thus effervescent drug formulations may be best avoided, or at least the tablet should be fully dissolved and allowed settle before drinking.


Patients should be monitored for any decreased efficacy or adverse effects, and changes made accordingly, rather than a blanket change in medicines after surgery. If absorption is reduced, a liquid preparation or an alternative administration route may resolve the problem.

Modifying formulations

The openings produced by certain types of bariatric surgery are small, so smaller formulations, or crushing or cutting tablets should be considered where appropriate, although this will make their use off-label.

Drug Administration Route

Alternative drug delivery routes may be considered. It is important to note that obesity may impact on drug absorption via the subcutaneous and transdermal routes.

Adverse Drug Reactions (ADRs)

Gastro-intestinal ADRs may be more likely in patients who have undergone bariatric surgery, due to the smaller size of the stomach.

Non-steroidal anti-inflammatory drugs (NSAIDs) should be avoided where possible as patients are more at risk of ulceration, which could be fatal.

ADRs should be reported via the MHRA’s Yellow Card Scheme https://yellowcard.mhra.gov.uk.

Further information

The effect of bariatric surgery (gastric bypass) on certain medicines

There is some evidence on the effect of bariatric surgery (gastric bypass) on specific medicines which may inform prescribing decisions for those patients.

Change history

  1. Fixed link to BOMSS guidelines on nutritional deficiencies
  1. Published