A research was conducted to study the reaction of Viagra with other drugs. The reaction was studied in-vitro (artificial, controlled environment outside the body of an animal) and in-vivo (in a living animal).  In the vitro studies it was found that sildenafil can control the action of the cytochrome P450 isoforms 1A2, 2C9, 2C19, 2D6, 2E1 and 3A4 (IC50 > 150 µM). But the action was weak as the plasma concentration of sildenafil is weak.

Studies on humans-
The study was conducted at three different stages. To ensure the most authentic results, the studies were double blind and randomized. Researchers attempted to study the reaction (if any) between doxazosin and Viagra. Doxazosin is an alpha blocker which is used for treatments of high blood pressure and benign prostatic hyperplasia.

Study I
In the first study, four men who were generally healthy but suffered from benign prostatic hyperplasia were divided into tow groups

Group I - Viagra + placebo
Group II - Viagra + doxazosin

Observations-

1. One of the patients reported a change on the postural hypotension (Decrease in blood pressure after a change in posture) after 35 minutes of taking Viagra which lasted for 8 hours.
2. Two others reported dizziness, headache and fatigue post an hour of the drug administration. This was also accompanied by lightheadedness and nausea, after 4 hours of taking the drug. None of there patients lost consciousness

These subjects were observed for 14 day continuously after being given daily doses. Post the review, the dose was decreased to 25 mg.

Study II –

• In this study, initially only a single oral dose of Viagara 50 mg was administrated to 20 healthy subjects who suffered from benign prostatic hyperplasia. After 14 days 17 subjects were giving doxazosin 4 mg and 3 subjects were given doxazosin 8 mg.
• Of these numbers, 19 subjects were put on a combination of matching placebo with Viagra 50 mg. One of the patients took minoxidil which is a vasodilator along with Viagra 50 mg due to which he had some adverse events and had to discontinue the study.

Observation- Among the patients who took the Viagra along with the placebo, the following observations were made

• Two patients reported a standing SBP of < 85 mmHg. This was reported as moderately severe adverse event. It started at approximately 1 hour post taking the drug and lasted for 7.5 hours
• One patient reported a drop from baseline in standing systolic BP > 30mmHg following VIAGRA 50 mg
• One patient reported drop from baseline in standing systolic BP > 30 mmHg following both VIAGRA 50 mg and placebo.

This study noted that none of the patients loss consciousness or reported any events with regards to their blood pressure.

Study III
This study involved 20 generally healthy males who suffered from benign prostatic hyperplasia. In this study each patient was administered open-label doxazosin along with a dose if Viagra 50 mg for 14 days continuously. The subjects who did not pass through this 14 day period successfully were discontinued from further studies. Post 14 days, the patients were continued to be given doxazosin for at least 7 days. After theses 7 days, patients were put on done of Viagra 100 mg with placebo along or with doxazosin 4 mg or doxazosin 8 mg (cross over studies)

The effect of this was studied on 20 subjects. 13 of these had successfully completed the first does period of 14 days and seven had completed the earlier doxazosin study.

The study resulted in the following observations.

Observations

• 3 subjects who were taking Viagra 100 mg had a standing SBP of < 85 mmHg. They also reported vasodilatation and dizziness.
• 4 subjects reported decrease from baseline in standing systolic BP > 30mmHg following VIAGRA 100 mg
• 1 subject reported a decrease from baseline in standing systolic BP > 30 mmHg following placebo
• 1 subject with a decrease from baseline in standing systolic BP > 30 mmHg following both VIAGRA and placebo

This study did not observe any severe adverse event relating to blood pressure and losing consciousness.

Some of the other interactions of Viagra and other drugs

• VIAGRA 100 mg (oral) + amlodipine, 5 mg or 10 mg (oral) à reduction on supine blood pressure was 8 mmHg systolic and 7 mmHg diastolic
• VIAGRA 100 mg (oral) + tolbutamide (250 mg) or warfarin (40 mg) (oral) à no significant interaction
• VIAGRA (50 mg) (oral) did not increase the bleeding time caused by aspirin (150 mg)
• VIAGRA (50 mg) did not increase the hypotensive effect of alcohol in healthy males with an average alcohol level of 0.08 %.
• VIAGRA 100 mg (oral) in healthy males did not affect the state of steady state pharmacokinetics of the HIV protease inhibitors, saquinavir and ritonavir.

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