If your testosterone levels are too low, it’s possible that Viagra and similar prescription ED medications might not work. That’s because PDE-5 inhibitors like Viagra work to increase blood flow to the penis and prepare the penis to receive that blood. But men with low testosterone might develop problems with their sex drive that prevent them from getting excited enough to make use of Viagra’s specific mechanisms. If you’re looking to testosterone boosters to treat ED but you aren’t sure whether you even have low testosterone, you might want to take an at-home testosterone test. If your numbers come back normal, then check out pills like Vigra or Cialis.