Pharmaceutical drug administration can be a very complex science in the obese population. Recommended medication doses are based on normal-weight patients, which can make dosing difficult when considering the physiologic changes related to both obesity and weight loss surgery. Studies have suggested that increased fat deposition can potentially affect the drug elimination and distribution. There are multiple different ways to describe a patient’s weight. These include total body weight (TBW), ideal body weight (IBW), adjusted body weight (ABW), and lean body mass (LBM) [1].

Total body weight (TBW) = patient’s actual weight

Adjusted body weight (ABW) = IBW + 0.4 (actual weight – IBW)

*ABW is only used if the actual body weight is >30 % of the calculated IBW.

Many pharmacologic parameters used to describe medications can vary in the obese patient. These include volume of distribution, clearance, and protein binding. The volume of distribution of substances that are highly lipophilic (fat loving) may be significantly increased in the obese population, e.g., benzodiazepines and barbiturates [1].

It has been suggested that obesity can alter the elimination and tissue distribution of medications as well. Maintenance and loading doses of specific medications that require narrow therapeutic windows may need adjustments in the obese population. In bariatric patients, this is an important consideration regarding antibiotics [2]. In a normal-weight individual, the blood flow in fat accounts for only 5 % of the cardiac output. The remaining output is distributed as 22 % to the lean tissue and 73 % to the viscera [3]. Increase in fatty tissue may therefore result in modified hemodynamics and pharmacokinetics of specific medications.

One other consideration to make is that many drugs are metabolized by cytochrome P450. This is a system of enzymes from the liver that function in biotransformation and oxidation of some drugs. Obese patients frequently have fatty deposition in their livers which may alter drug metabolism in a way that is not yet fully understood [2, 4]. Unfortunately, this area of research is still underdeveloped and requires more investigation before definitive recommendations on dosing in obesity can be made.

Some medications do have recommended weight adjustment parameters that we can use for dosing [1, 5]:

With regard to drug absorption, the proximal fourth of the intestines accounts for half of the total mucosal surface of the gut. This portion also has the greatest capacity for absorption. When considering Roux-en-Y gastric bypass surgery, there are both restrictive and malabsorptive components of weight loss. This could result in decreased bioavailability of drugs that have delayed absorption. Reducing the stomach to a pouch causes decreased hydrochloric acid production and therefore slightly higher pH. Medications that depend upon gastric ionization may be altered as well [6].