Farmacia El Piri on Calle 1 across from the wax Museum has large stock of meds and probably the…read morelowest prices in Mexico.
They sell Ultra's Sildenafil 100 mG, 2 boxes with 16 pills, for 90 pesos or $4.75 or about 30 cents per pills. That brand works well with little to no side effects. I normally take only 1/8 pill or 10 mG for extra duty, never failed.
Other Sildenafil and Tadalafil brands are available at higher prices. I would stick to the cheap Ultra brand, 2 boxes for $5, 16 pills chopped into 8 pcs each, 128 slivers of 10 mG, lasts me 128 weekends or 2 years.