ddI (didanosine, Videx / VidexEC)
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Many other medications can reduce the amount of ddI (didanosine, Videx / VidexEC) that the body absorbs. ddI tablets contain an antacid to prevent the drug being destroyed by stomach acid. Drugs that require an acidic stomach for proper absorption should be taken at least two hours before ddI tablets. These include:

  • Ciprofloxacin (Ciproxin).
  • Dapsone.
  • Itraconazole (Sporanox).
  • Ketoconazole (Nizoral)
  • Levofloxacin (Tavanic).
  • Moxifloxacin (Avelox).
  • Naldixic acid (Uriben).
  • Norfloxacin (Utinor).
  • Orfloxacin (Tarivid).

ddI capsules may be taken at the same time as these drugs.

ddI tablets should not be taken at the same time of the day as any protease inhibitors as they may prevent the protease inhibitors from being properly absorbed. Patients taking ddI and a protease inhibitor should take the protease inhibitor at least two hours before the ddI tablets. Indinavir (Crixivan) may be particularly difficult to schedule, since both drugs have to be taken on an empty stomach. Even if indinavir is being taken with ritonavir (Norvir), thus avoiding food restrictions, it must not be taken within two hours of ddI tablets.

There is no need for the two-hour gap between indinavir (Crixivan) and ddI capsules, although protease inhibitors which must be taken with food should be not be dosed at the same time as ddI capsules.

If ddI is taken with tenofovir (Viread), levels of ddI can increase by up to 64%[1][2]. This can put patients at a high risk of side-effects such as pancreatitis and neuropathy. However, this combination of drugs can also cause dramatic falls in CD4 cell counts in up to 80% of patients, even when they have undetectable viral loads[3][4]. This is thought to be due to the very high levels of ddI leading to death of the white blood cells[5]. These paradoxical falls in CD4 cell count are more likely in patients who have been taking ddI for longer, with higher doses of ddI relative to weight and in patients with kidney damage[6]. Furthermore, this combination of NRTIs has been linked to virological failure and the development of resistance in numerous studies[7][8].

A dose reduction from 400 to 250mg ddI once a day may be considered for patients who need to take these two drugs together to construct a viable anti-HIV regimen, and close monitoring for side-effects and CD4 cell count declines is advised[9][10]. This was reflected in ‘Dear Doctor’ letters from Bristol-Myers Squibb and Gilead, tenofovir’s manufacturer, in November 2004 and March 2005, and a warning from the European Medicines Agency (EMEA) in April 2005.

Certain drugs can increase the risk of ddI-related pancreatitis and should be avoided by patients taking ddI. These include the following:

  • d4T (stavudine, Zerit).
  • Demeclocycline hydrochloride (Ledermycin).
  • Doxycycline (Vibramycin / Vibramycin-D).
  • Hydroxycarbamide (Hydrea).
  • Lymecycline (Tetralysal 300).
  • Minocycline (Minocin MR / Sebomin MR).
  • Oxytetracycline.
  • Pentamidine isetionate (Pentacarinat).
  • Tetracycline.

Alcohol should also be avoided by people taking ddI because it can increase the risk of pancreatitis. Similarly, rifampicin (Rifadin / Rimactane), rifabutin (Mycobutin), ganciclovir (Cymevene), interferon alfa (IntronA / Roferon-A / Viraferon), ribavirin (Copegus / Rebetol / Virazole), co-trimoxazole (Septrin) and some stomach ulcer drugs including H2-receptor blockers and omeprazole (Losec) may increase the risk of pancreatitis[11][12][13].

Methadone hydrochloride (Methadose) may seriously reduce absorption of ddI, particularly the tablet form. Studies are underway to investigate whether the capsule form of ddI is better absorbed in people taking methadone.