Implications of Bariatric Surgery on Absorption of Nutrients and Medications

Mary Carpenter, PharmD; Mary Ellen Pisano, PharmD; Christopher M. Bland, PharmD, BCPS, FIDSA


US Pharmacist. 2016;41(12):HS-2-HS-8. 

In This Article

Oral Bioavailability of Drugs

Availability of oral medications to the systemic circulation is dependent upon its absorption, also known as oral bioavailability. The rate and extent of absorption depend on the medication's disintegration, dissolution, and diffusion. Medications may also be metabolized by GI or hepatic enzymes before reaching systemic circulation. Prior to drug approval, oral bioavailability of medications is typically studied in patients with normal GI tracts. Bariatric surgery results in anatomical changes to the GI tract, and thus it can affect absorption and metabolism of oral medications. As mentioned previously, mechanisms include changes in gastric/intestinal pH, gastric emptying time/intestinal transit time, surface area available for absorption, active transport mechanisms for absorption, and first-pass metabolism.[25] Possible effects on the GI tract for the various procedures can be found in Table 1.

Utilizing dosage forms that can help reduce issues associated with the anatomical changes of the GI tract is paramount for bariatric patients. This applies primarily to patients who have undergone RYGB surgery. Dosage forms that are designed for normal GI tracts such as those requiring disintegration (solid dosage forms), acidic environments (enteric-coated and delayed-release products), prolonged dissolution (extended-release products), and slower diffusion (oily solutions, suspensions) should be avoided, if possible.[25] Aqueous solutions are the optimal dosage form because they are more rapidly absorbed and exempt from the disintegration and dissolution processes. If a drug is only available as a solid, the use of an immediate-release or crushing/chewing formulation is preferred. In addition, other routes of administration should be considered (if available) including IV, rectal, vaginal, intranasal, sublingual, transdermal, subcutaneous, and intramuscular.

An abundance of literature describing the proposed mechanisms of reduced medication absorption exists; however, research on the malabsorption of specific medications and patient outcomes associated with altered drug efficacy following bariatric surgery is sparse. The consensus of the available literature is that the effect of bariatric surgery on medication absorption appears to be drug-specific.[14,26]