Drug Interactions For Viagra Doctors Recommend To Avoid

In order to understand the effects of other drugs on Viagra (sildenafil citrate), one should keep in mind that sildenafil is metabolized primarily with the help of cytochrome isoform 3A4 and secondary with 2C9 isoform. When taking Viagra simultaneously with drugs that depress these isoferments it is important to understand that they will delay sildenafil clearance, and contrariwise, inductors of these isoforms will bring on increased clearance.

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How Do Other Drugs Affect Viagra?

Results of pharmacokinetic analysis yielded by clinical studies in vivo showed that drugs inhibiting CYP3A4 decrease clearance of sildenafil when taken alongside with Viagra. Drugs containing such inhibitors are ketoconazole, erythromycin, cimetidine. General recommendation is to dial down the initial dose of sildenafil to Viagra 25 mg as an initial dose and see how things play out when taking inhibitors CYP3A4 alongside with Viagra pills.

Viagra Drug Interaction

Viagra is affected by HIV protease inhibitors ritonavir, which depress P450 in a major way. When ritonavir drugs are taken in equilibrium concentration in quantity of 500 mg per day alongside with Viagra 100 mg pills in a singular taking, sildenafil Cmax (maximum systemic exposure to the drug’s effects) is increased by 300%, which is 4 times higher than normally. Plasma AUC of sildenafil reaches 1,000%, or 11 times higher. Plasma levels continue to be higher than normal 24 hours after 24 hours, which can be explained by ritonavir interaction with a broad spectrum of P450 substrata. Sildenafil contained in Viagra does not affect ritonavir pharmacokinetics. Combination of Viagra and ritonavir therapy is not recommended based on the above. When expected benefits outweigh potential risks, such combination is possible under condition that sildenafil dose does not exceed that of Viagra 25 mg taken once during 48 hours.

HIV protease inhibitor saquinavir (CYP3A4 inhibitor) in dose that provides equilibrium concentration (1,200 mg taken three times a day) affects sildenafil Cmax increase to 140% (data for Viagra 100 mg). The plasma concentration-time profile of sildenafil is also affected by saquinavir, since systemic exposure (‘area under the curve’) of Viagra is heightened by 210%. There has been no evidence of Viagra affecting saquinavir pharmacokinetics. It is assumed that more powerful CYP3A4 inhibitors like ketoconazole or itraconazole will have a more pronounced effect.

Sildenafil systemic exposure is increased to 182% (AUC) during simultaneous intake of erythromycin, which is a moderate inhibitor of CYP3A4, taken in equilibrium concentration doses (500 mg tablets taken twice a day during 5 days). During in vivo studies there was no increase in AUC, Cmax of Tmax aspects of sildenafil clearance during simultaneous intake of azithromycin in dose of 500 mg tablets 3 times a day. The same concerned sildenafil’s principal metabolite. Cimetidine  (an inhibitor of cytochrome P450 and a nonspecific inhibitor of CYP3A4) can lead to Cmax increase by 56% when concomitantly taken with Viagra 50 mg tablets.

Sildenafil clearance time may be increased by grapefruit juice, since this latter is a mild inhibitor of CYP3A4 in small intestine.

Bioavailability of sildenafil was not affected by simultaneous use of antacids like magnesium hydroxide / aluminum hydroxide.

Although a full picture of Viagra interaction with every drug from the group of CYP2C9 inhibitors, there is enough evidence to say that sildenafil bioavailability and kinetics do not get affected when taken together with the drugs of this group. Thus, Viagra effects are not altered in any way when the drug is taken alongside with tolbutamide, warfarin, phenytoin, as well as CYP2D6 inhibitors, including selective serotonin reuptake inhibitors and tricyclic antidepressants, groups of thiazide diuretics and drugs similar to thiazide, loop and potassium-sparing diuretics, angiotensin-converting enzyme inhibitors, calcium antagonists, β-adrenoceptor antagonists or inducers of CYP450 metabolism (such as rifampicin and barbiturates).

Simultaneous use of Viagra in equilibrium concentration doses of 80 mg three times a day and inductors of CYP3A4, CYP2C9 and, partially, CYP2C19 taken in equilibrium concentration doses of 125 mg twice a day leads to decrease in quantative AUC aspects of sildenafil by 62.6% and 55.4% respectively. There may be observed a more pronounced drop in sildenafil clearance in plasma when Viagra is taken alongside with rifampin and other powerful inductors of CYP3A4.

Nikorandyl is a hybrid calcium channel activator and nitrate. Nitrate component of the drug determines the possibility of serious interaction with sildenafil.

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How Does Viagra Affect Other Drugs?

Sildenafil is a weak inhibitor of 1А2, 2C9, 2C19, 2D6, 2E1 isoferments and ЗА4 cytochrome of Р450, but since Cmax values of Viagra are insignificantly low, the drug’s influence on clearance of substrata of these isoforms is improbable. There is no data on non-specific inhibitors of phosphodiesterase as dipyridamole and theophylline.

Viagra Drug InteractionSince it has been long known that sildenafil affects metabolism of nitric oxide / cyclic guanosine monophosphate (cGMP), it was has been concluded that Viagra can potentiate hypotensive action of nitrates. It is therefore prohibited to use Viagra alongside with any other pharmacological products containing nitrates.

If a person is predisposed to low blood pressure, taking Viagra together with alpha-blockers can lead to symptomatic hypertension. This reaction is reported to have normally happened within 4 hours after the use of sildenafil. Individuals with benign prostatic hyperplasia who receive treatment with an α-blocker called doxazosin (4 mg and 8 mg) experience significant drop in blood pressure while simultaneously taking Viagra 25 mg, Viagra 50 mg and Viagra 100 mg. There have been reports about symptomatic orthostatic hypotension that are accompanied by pre-syncope states and dizziness, but no syncope instances have been reported.

Drugs that are metabolized with the help of CYP2C9 isoferment (tolbutamide 250 mg) or warfarin 40 mg have not been observed to come in any significant interaction with Viagra.

Sildenafil contained in Viagra 50 mg did not extend time of haemorrhage brought on by the use of acetylsalicylic acid.

Alcohol contained in blood with average maximal quantities of 80 mg/dl does not interact with sildenafil effects.

There have been observed no deviation from standard side event profile in patients using Viagra alongside with the application of such classes of antihypertensive drugs as diuretics, blockers, beta-blockers, ACE inhibitors, antagonists of angiotensin II, antihypertensive medicinal products (vasodilator and centrally acting), blockers of adrenergic neurons, calcium channel blockers and alpha-blockers.

Sildenafil 100 mg did not influence the pharmacokinetic parameters of HIV protease inhibitors, saquinavir and ritonavir, which are substrates of CYP3A4.

Bosentan 125 mg taken alongside with Viagra (80 mg three times a day) can have increased clearance time (AUC and Cmax) for 42% 49.8% respectively.