Dosing & Drug Interaction Study of Viagra, Cialis and Levitra

Dosing

Patient Type	Viagra	Cialis	Levitra
Unrestricted	Patients are usually advised to start with 50mg dose an hour before sexual activity. Range: 25-100 mg doses	10mg starting dose is recommended and dose is optimized based on patient response. Can be given the full range 2.5mg-20mg	Patients are usually advised to start with 10mg dose before sexual activity. Can be recommended full range: 5-20mg mg.
Patients over 65	Recommended starting dose of 25mg and upped if necessary.	No dose adjustment necessary. Same as 'unrestricted'	Recommended starting dose of 5mg.
Patients with liver problems	Recommended starting dose of 25mg and upped if necessary.	Mild to moderate impairment: Usually best not to exceed 5mg. Severe liver problems: Not Recommended	Recommended starting dose of 5mg.
Patient with kidney problems	Recommended starting dose of 25mg and upped if necessary.	Dosage not to exceed 10mg	No dose adjustment necessary.

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.