Some drugs, especially opiates such as morphine and codeine, slow the digestive process and may alter the absorption rate of a drug. Conversely, other drugs, such as metoclopramide, speed up the digestive process and may affect the speed and extent to which a drug is absorbed.

Calcium is one drug that interacts with a range of medications. It can interfere with the absorption of tetracyclines and quinolone antibiotics and diminish their effectiveness. When taken with corticosteroids, it is the effect of calcium that is lessened. Combined with an oestrogen, absorption of calcium increases, thus helping improve bone density. The effects caused by calcium can generally be avoided by spacing intake of the interacting drugs several hours apart. It is a good idea to check with a pharmacist regularly for potential drug interactions and to also check in whenever a drug is added or removed from the daily regimen.  

An important interaction with the original formulations of ddI (didanosine, Videx) concerns stomach acidity. Didanosine absorption after oral administration is variable due to its poor solubility in an acidic environment. For this reason, citrus juices such as orange or grapefruit should not be taken at the same time as ddI. Conversely drugs that lower stomach acidity will increase ddI absorption and possibly also increase its toxicity. Examples of drugs that do this are cimetidine (Dyspamet/Tagamet), ranitidine (Zantac) and antacids such as Rennies and magnesium trisilicate. The original formulation of ddI is now formulated with a buffer that neutralises stomach acid and allows the ddI to be absorbed; this will have a negative effect on other drugs that do need stomach acid to be absorbed, including dapsone, ketoconazole (Nizoral), and ciprofloxacin (Ciproxin). The re-formulated version of ddI (enteric coated; VidexEC), which received licensing in 2000, does not require the buffer used in the previous formulation. ddI EC consists of gastro-resistant capsules containing enteric coated beadlets of ddI.

A similar interaction can occur between protease inhibitors and acid-reducing agents, such as ranitidine. Since many protease inhibitors need the stomach to be acidic in order to allow proper drug absorption into the bloodstream, the use of acid reducing agents can reduce blood levels of protease inhibitors. This in turn may reduce the anti-HIV activity of the protease inhibitor affected by this type of interaction. A number of acid-lowering agents are available without prescription and over-the-counter in chemists, so many patients may be unaware of these risks when they buy drugs to treat heartburn or indigestion.

The protease inhibitor most affected by acid-reducing agents is atazanavir (Reyataz). The extent of its interactions with different acid-reducing agents is still being explored, so caution and consultation with a pharmacist is advised if planning to take atazanavir (or other protease inhibitors) with an acid-reducing agent.