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

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

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

Cardiovascular

Amiodarone

Bioavailability may be reduced by shortened intestinal transit time.

Monitor and adjust dose as required.

Aspirin 75mg daily dose

There is no data on safety so use should be based on an individual risk versus benefit decision.

Digoxin

Digoxin absorption is dependent on both GI transit time and P-glycoprotein in the small intestine. The extent of absorption may therefore be reduced.

Monitor and adjust dose as required.

Enalapril

The prodrug is converted to the active form in the stomach so efficacy may be reduced. Consider an alternative ACE Inhibitor if problematic.

One case report describes the death of a woman due to complete heart block secondary to enalapril toxicity 11 days after undergoing bariatric surgery. The authors suggest that bypassing pre-systemic cytochrome P450 enzymes in the proximal short bowel led to a larger amount of free drug available.

Metoprolol

Likely to be absorbed in the stomach and duodenum.

Monitor blood pressure and adjust dose accordingly. Dose may need to be reduced as weight loss occurs.

Ramipril

Absorption is decreased in patients with steatorrhoea and malabsorption. It is likely to also be decreased in patients who have undergone bariatric surgery. Consider using another ACE inhibitor if a lack of efficacy is problematic.

Dose may need to be reduced as weight loss occurs.

Simvastatin

Likely converted into active form in the stomach. Efficacy may be reduced. Consider an alternative statin.

Monitor serum lipids.

Warfarin

The effects of bariatric surgery may be unpredictable: reduced surface area may limit absorption. However a more alkaline stomach may increase levels of unionised drug. Vitamin K deficiency may lead to an increased risk of bleeding.

Monitor closely and adjust dose as required.

Endocrine

Bisphosphonates

Increased risk of GI adverse effects – avoid where possible.

However, patients who have undergone bariatric surgery may also be at an increased risk of osteoporosis. Consider other options such as calcitonin salmon nasal spray, teriparatide, raloxifene etc. where clinically suitable, or an IV bisphosphonate.

Levothyroxine

Patients who have undergone Roux-en-Y surgery may require a reduction in levothyroxine dose after surgery.

Monitor via regular thyroid function tests for 6 months post-surgery, with dose adjustment as required.

Metformin

Absorbed slowly in the duodenum.

Monitor blood glucose and adjust dose accordingly. Dose may need to be reduced as weight loss occurs.

Gastrointestinal

Acid suppression (proton pump inhibitors/ H2 blockers)

The use of acid suppressive agents may lead to a delay in achievement of peak weight loss following surgery. It is not currently known why this occurs.

The extent of weight loss in the long-term does not appear to be affected.

Infections

Fluoroquinolones

Absorption is via transportation in the small intestine. Absorption may be reduced.

Ketoconazole

Requires an acidic environment for absorption. Consider an alternative antifungal.

Mental health

Olanzapine

Absorbed in the stomach, even if administered in orodispersible form.

Monitor for decreased efficacy. Adjust dose accordingly or consider switching to another antipsychotic.

Quetiapine

Likely to be absorbed in the stomach and duodenum.

Monitor for decreased efficacy. Adjust dose accordingly or consider switching to another antipsychotic.

Zolpidem

Absorption may be delayed. Take on an empty stomach.

Neurology

Lamotrigine

This is likely to be absorbed in the stomach and proximal small intestine.

Monitor for decreased efficacy.

Valproic acid/ Sodium valproate

The extent of absorption valproic acid may be significantly reduced by malabsorptive procedures. It is currently unknown if sodium valproate is similarly affected.

Monitor closely and adjust dose accordingly.

Obstetrics and Gynaecology

Contraceptives

The effectiveness of oral contraception (OC), including oral emergency contraception (EC) could be reduced by bariatric surgery, and OC should be avoided in favour of non-oral methods of contraception.

Non-hormonal barrier contraception should be recommended as reduced absorption cannot be easily monitored. If a hormonal method is required, a levonorgestrel-releasing intrauterine device may be a preferred option.

Oestrogen-containing contraceptives should be avoided before and after surgery to reduce the risk of venous thromboembolism.

A reduction in weight following surgery may also lead to increased fertility.

Pain

Non-Steroidal Anti-inflammatory Drugs

There is an increased risk of GI adverse effects so the advice is to avoid where possible.

Further information

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

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

Change history

  1. Additional contraceptive information from Faculty of Sexual & Reproductive Healthcare (FSRH) 'Overweight, Obesity and Contraception' (April 2019) added.
  1. Published