In a study of healthy male volunteers, sildenafil mg did not affect the steady state pharmacokinetics of the HIV protease inhibitors, saquinavir and ritonavir, both of which are CYP3A4 substrates. Similar values for pharmacokinetic parameters were seen in normal volunteers and in the patient population, using a population pharmacokinetic approach. If you have an erection that lasts for more than 4 hours, you should go to the nearest hospital emergency department. This will allow your doctor to see if the medicine is working properly and to decide if you should continue to take it. Children 1 year of age and older weighing 45 kilograms kg or more—20 mg 3 times a day.PHUKET: Viagra will soon be sold in Phuket pharmacies following a decision by Thailand ’s Food and Drug Administration (FDA). The drug was previously only available through hospitals or illegally on the black market. 11 years ago. Save. I’ve just read this on another forum about Viagra and Thailand; It is a controlled substance and legally cannot be purchased without a Prescription. It is illegal to have it in your posession without a prescription as well. Not sure if . Examples of Viagra’s more commonly reported side effects include: headache. flushing (temporary warmth, redness, or deepening of skin color) stuffy nose. indigestion (upset stomach) mild vision.