The importance of considering medicines and bariatric surgery

Weight loss surgery, sometimes called bariatric surgery, is recommended by NICE  as a treatment option for obese patients who meet specific criteria. The main types of weight loss surgery are gastric bypass, gastric band, gastric balloon and sleeve gastrectomy.

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 the number of patients eligible for bariatric surgery will also increase. Therefore, an understanding of the likely effect bariatric surgery can have on medicines is increasingly important.

Considerations

The effect of bariatric surgery on the pharmacokinetics of medicines is complex, and will also be influenced by the type of surgery.

In general, oral medicine absorption decreases due to gastric restriction or intestinal bypass, among other physiologic alterations. There is also a reduction in the bioavailability of medicines which rely on food for their absorption.

Changes should not be made to medicines based on just one of these effects. All patients should have frequent monitoring to identify decreased efficacy or adverse effects, particularly for medicines with a narrow therapeutic index. Changes should be made individually, rather than generalised changes to medicines after surgery.

Medicines may need to be given via a different route, formulations may need to be modified, or doses may have to be adjusted accordingly.

Medicines specific advice

See our article if you need medicines specific advice following bariatric surgery.

• Specific advice for using medicines following bariatric surgery

Bariatric surgery may effect specific medicines. Guidance is given to inform prescribing decisions for those patients.

Stomach size

The size of the stomach is reduced through bariatric surgery. 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 gastric pouch could therefore lead to a less complete disintegration and affect absorption.

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.

Conversely, the more alkaline environment may increase the absorption of some medicines. Although those that are soluble in alkaline environments are mainly absorbed in the small intestine.

In restrictive procedures (such as sleeve gastrectomy) and malabsorptive procedures (such as gastric bypass) the majority of the small intestine may be bypassed. This can lower the absorption of medicines in that area.

Surface area

Bariatric surgery significantly reduces the amount of surface area in the gastrointestinal tract available for medicine absorption. There is also a quicker transit time.

Medicines that stay in the intestine longer during absorption, or are mainly absorbed in the duodenum and jejunum, may have lower bioavailability.

Drug formulation

The ability to absorb medicines is changed by bariatric surgery. However, the exact change in absorption is medicine and procedure specific. The opening size between the pouch and the stomach produced by certain types of bariatric surgery such as a gastric band may be only 1 to 1.5cm wide. Therefore some dosage forms may get stuck and remain undissolved.

Modifying formulations

In the first few weeks after surgery liquid or crushed preparations may be required. However, the volume of liquid also needs to be considered in light of the reduced stomach capacity. Smaller formulations, which may be achieved by cutting tablets, should be considered where appropriate. Cutting or crushing tablets may make their use off-label. We advise on how to check if tablets can be crushed in our article.

If liquid preparations are used they should be low sugar to reduce the risk of dumping syndrome. This occurs when food is emptied too quickly into the small intestine, filling it with undigested food which causes inefficient absorption.

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

After 6 to 8 weeks, once back on a normal diet, return to solid dosage forms. Consider staggering doses due to the reduced capacity of the stomach.

Release characteristics

Immediate release formulations are preferred. Extended release, enteric, or film coated formulations should be avoided where possible, as any delay in drug release may reduce absorption. This is not an issue in patients who have had gastric bands or balloons.

Effervescent formulations

Carbonated beverages often cause discomfort after surgery. Therefore effervescent drug formulations should be avoided. If they cannot be avoided, allow the tablet to fully dissolve and settle before drinking.

Supplements

There is a high prevalence of nutritional deficiencies in those with severe and complex obesity. Bariatric surgery impacts both oral intake and absorption, raising the risk of nutritional deficiencies. However, restrictive procedures, such as sleeve gastrectomy, are less likely to cause nutrient deficiencies.

Regardless of the procedure, patients will be required to stay on lifelong nutritional supplements and have lifelong monitoring of their nutritional status after surgery.

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

Further guidance including what monitoring is required can be found in the British Obesity and Metabolic Surgery Society post-baratric surgery nutritional guidelines. NICE provides specific guidance regarding vitamin B12 supplementation.

Drug administration route

Alternative drug delivery routes may be considered such as intramuscular, intravenous, subcutaneous and transdermal routes. Obesity may also impact on drug absorption via the subcutaneous and transdermal routes.

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