Note that it is illegal to fail to declare imported medications with CBP agents properly. Sildenafil usually should not be taken more than once every 24 hours. This increased blood flow can cause an erection. This is especially true with drugs that are available only by prescription in the United States but sold over the counter in Mexico. Follow the manufacturer’s directions to use and clean the oral syringe. It also explains the legality of buying medications in Mexico and the rules regarding crossing the border with pharmaceuticals.Typical dosing for Sildenafil (Viagra) The typical dose is to take 50 mg of sildenafil (Viagra) by mouth, about 1 hour before sex, as needed. The maximum recommended dose is mg. Don’t take sildenafil (Viagra) more than once daily (at any dose). Written by Brian Leonard, PharmD, BCACP, BCGP. Viagra (sildenafil) is a prescription drug that’s used to treat erectile dysfunction (ED). The drug comes as a tablet you’ll take by swallowing. It’s usually taken as needed about 1 hour. Updated on July 17, Medically reviewed by Mary Choy, PharmD. Fact checked by Heather Mercer. Table of Contents. Prescriptions From Mexico. When Is It Legal to .