How they both work
Sildenafil (Viagra generic) and tadalafil (Cialis generic) are both PDE5 inhibitors. They block the enzyme that breaks down cGMP, allowing smooth muscle in penile tissue to relax and blood flow to increase. Neither causes an erection without arousal — they lower the threshold for a natural response.
Onset time
Sildenafil reaches peak plasma concentration in 30–60 minutes. Taking it on an empty stomach speeds absorption; a high-fat meal delays it by up to 60 minutes. Tadalafil peaks in 2 hours but the difference in clinical effect is modest — most men feel onset within 30–45 minutes regardless.
Duration of action
This is where they diverge sharply. Sildenafil has a half-life of 3–5 hours; the window of effect is roughly 4–6 hours. Tadalafil has a half-life of 17.5 hours, giving an effective window of 24–36 hours. This longer window gives tadalafil its “weekend pill” reputation and makes low-dose daily use (5 mg) practical.
Food and alcohol interaction
Sildenafil: avoid grapefruit juice (CYP3A4 inhibitor) and high-fat meals before dosing. Moderate alcohol (≤2 units) is generally acceptable but increases the risk of low blood pressure. Tadalafil: food has minimal effect on absorption. The same alcohol caution applies.
Side-effect profile
Both share a class-effect profile: headache, flushing, nasal congestion, dyspepsia. Tadalafil additionally causes back pain and myalgia in ~3–5% of users, likely due to its effect on PDE11 in muscle tissue. Sildenafil causes more pronounced visual disturbance (blue tinge, light sensitivity) due to PDE6 cross-reactivity — more noticeable at higher doses.
Which to choose
If spontaneity matters and you want a long window: tadalafil. If you prefer a shorter, more predictable window and lower cost: sildenafil. Both are equally effective when taken correctly.