Levitra is almost 10 times more biochemically potent than sildenafil; therefore, a lower dose is needed to facilitate the penile erection, potentially resulting in fewer side effects. Selectivity refers to the ability of a drug to bind specifically to the PDE5 enzyme found in penile tissues. The maximum recommended dosing frequency is once per day. Sexual stimulation is required for a response to treatment. LEVITRA can be taken with or without food.