Sildenafil (Viagra) – Interactions and Warnings

Sildenafil (Viagra)
– Interactions and Warnings

These results indicate that both saquinavir and ritonavir modify the pharmacokinetics of sildenafil. Given the extent of the interactions, a lower sildenafil starting dose (25 mg) should be considered for patients receiving saquinavir and it is recommended not to exceed a maximum single dose of 25 mg in a 48 h period for patients receiving ritonavir.
– Br J Clin Pharmacol 2000 Aug;50(2):99-107 — Pharmacokinetic interactions between sildenafil and saquinavir/ritonavir. — Muirhead GJ, Wulff MB, Fielding A, Kleinermans D, Buss N.

Some findings indicate that in addition to established contraindications (allergy against the active ingredient, concomitant nitrate and NO-donor treatment, retinitis pigmentosa) factors such as coronary heart disease, advanced age, impaired liver function and drug interactions require special attention by the prescribing physician to secure drug safety.
– Urologe A 1999 Mar;38(2):124-7 — Sildenafil (Viagra). Tolerance, contraindications, drug interactions — Sperling H, Michel MC, Rubben H.

Sildenafil produces mild decreases in systolic and diastolic blood pressure and an array of minimal side effects due to the inhibition of other types of phosphodiesterase. Drug interactions involving the concurrent use of sildenafil with nitrates and nitrites are well-documented and can produce profound hypotension leading to decreased coronary perfusion and myocardial infarction.
– J Toxicol Clin Toxicol 2000;38(6):645-51 — Sildenafil: clinical toxicology profile. — Krenzelok EP.

Potential drug interactions with cimetidine, protease inhibitors, some antihypertensive drugs, alcohol, and drugs that may competitively inhibit cytochrome P450 pathways.
– Ann Pharmacother 2001 Mar;35(3):337-42 — Is the sildenafil product information adequate to facilitate informed therapeutic decisions? — Cohen JS.

Possible interactions: Use caution with the following herbs: Yohimbine, Citrulline, two pyrano-isoflavones, berberine, forskolin.
– Phytochemistry 2003 Apr;62(7):1019-25 — Recent findings on natural products with erectile-dysfunction activity. — Drewes SE, George J, Khan F.

Possible interaction: Use caution with korean red ginseng.
– J Fam Pract 2003 Jan;52(1):20-1 — Korean red ginseng effective for treatment of erectile dysfunction. — Price A, Gazewood J.

Sildenafil is metabolized in the liver through cytochrome P-450. This enzymatic system can be inhibited by cimetidine, ketoconazole or erythromycin. These drugs can increase plasma concentrations of sildenafil.
– Rev Med Chil 1998 Nov;126(11):1285-7 — Sildenafil (viagra) at the time of warnings — Prieto Dominguez JC.

Study shows that oral sildenafil is a potent pulmonary vasodilator that acts synergistically with inhaled iloprost to cause strong pulmonary vasodilatation in both severe pulmonary arterial hypertension and chronic thromboembolic pulmonary hypertension.
– Ann Intern Med 2002 Apr 2;136(7):515-22 — Combination therapy with oral sildenafil and inhaled iloprost for severe pulmonary hypertension. — Ghofrani HA, Wiedemann R, Rose F, Olschewski H, Schermuly RT, Weissmann N, Seeger W, Grimminger F.

Adverse events considered to be related to sildenafil treatment included headache, nausea, and dyspepsia. In patients with hypertension who were taking amlodipine therapy, sildenafil produced additive, but not synergistic, reductions in blood pressure.
– Am J Cardiol 1999 Mar 4;83(5A):21C-28C — Sildenafil citrate and blood-pressure-lowering drugs: results of drug interaction studies with an organic nitrate and a calcium antagonist. — Webb DJ, Freestone S, Allen MJ, Muirhead GJ.

The concomitant use of sildenafil and different classes of antihypertensive agents (beta-blockers, alpha-blockers, diuretics, ACE inhibitors, calcium antagonists) may lead to additive but not to potentiating blood pressure decreases.
– Dtsch Med Wochenschr 2001 Oct 12;126(41):1144-9 — Interaction between sildenafil and antihypertensive drugs: what is evidence-based? — Rosenkranz S, Erdmann E.

Indinavir inhibition of the hepatic metabolism of sildenafil.
– AIDS 1999 Oct 22;13(15):F101-7 — Interaction of sildenafil and indinavir when co-administered to HIV-positive patients. — Merry C, Barry MG, Ryan M, Tjia JF, Hennessy M, Eagling VA, Mulcahy F, Back DJ.

Grapefruit juice increases sildenafil bioavailability and tends to delay sildenafil absorption. Sildenafil pharmacokinetics may become less predictable with grapefruit juice. Although patients usually will not be endangered by concomitant use of grapefruit juice, it seems advisable to avoid this combination.
– Clin Pharmacol Ther 2002 Jan;71(1):21-9 — Effects of grapefruit juice on the pharmacokinetics of sildenafil. — Jetter A, Kinzig-Schippers M, Walchner-Bonjean M, Hering U, Bulitta J, Schreiner P, Sorgel F, Fuhr U.

There may be an interaction with St. John’s Wort and Sildenafil. Consult with your physician.

last update: November 2008

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