Viagra generally refers to sildenafil citrate tablets, which are mainly used to treat erectile dysfunction and can be taken orally 1 hour before sex. The effect of the drug can last for about 4 hours after taking sildenafil citrate tablets orally, and it takes effect about 0.5~1 hour after taking the drug. So it’s okay to take it orally 1 hour before sex, and it’s also the recommended time to use it. However, it should be used on an empty stomach to avoid mixing with food to delay the onset of the drug. In addition, if you are using nitrate antihypertensive drugs people can not take sildenafil orally, to avoid causing low blood pressure. Sildenafil citrate tablets are prescription drugs, some patients may experience dizziness, headache, nausea, vomiting and other uncomfortable symptoms after using the drug, if necessary, need to go to the hospital in time for examination and treatment.