Dosing & Drug Interaction Study of Viagra, Cialis and Levitra

Dosing

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Drug Interaction

Viagra, Cialis and Levitra are all metabolized by CYP3A4 and this means that medicines that control or affect this particular isozyme can potentially inhibit the clearance of all 3 PDE5 inhibitors in question.

So what medicines contain CYP3A4 inhibitors? Medicines used to treat HIV infection such as ritonavir and amprenavir, anti-fungal medicines like ketoconazole and itraconazole and even antibiotics like erythromycin contain CYP3A4 inhibitors.

It is commonly known that Viagra, Cialis and Levitra should not be taken with anitrates. If you are taking nitrate medications(a common angina medication), it is generally advised to not take both at the same time. If one is using nitrates and wants to use Viagra – it is best advised to leave a gap of atleast 24 hours from the last nitrate/alpha blocker dose before trying Viagra. Likewise nitrate medication should not be resumed till atleast 24 hours have passed after taking Viagra or Levitra and 48 hours have passed after taking Cialis.

Viagra can also interact with alpha blockers usually given for hypertension and certain other conditions. Ideally a gap of 4 hours before the last alpha blocker drug must be maintained before consuming Viagra. However, lower doses of Viagra and Cialis are usually tolerated without the time gap.