Short answer: Both are PDE5 inhibitors for erectile dysfunction. Sildenafil (Viagra) is usually 50 mg about 1 hour before sex, with a half-life of about 4 hours.1 Tadalafil (Cialis) is 10 mg as needed, with a half-life of 17.5 hours and effect up to 36 hours; it can also be taken daily and is approved for BPH.2
| Tadalafil (Cialis) | Sildenafil (Viagra) | |
|---|---|---|
| Approved for | ED; BPH; ED with BPH2 | ED1 |
| Label dose (ED, as needed) | 10 mg before sexual activity; 5 to 20 mg; at most once a day2 | 50 mg about 1 hour before (anywhere from 30 minutes to 4 hours); 25 to 100 mg; at most once a day1 |
| Daily option | 2.5 to 5 mg once daily2 | None on the label |
| Time to peak | Median 2 hours (30 minutes to 6 hours)2 | Median 60 minutes (30 to 120) fasting1 |
| Half-life | about 17.5 hours (label: 17.5 hours)2 | About 4 hours (sildenafil and its active metabolite)1 |
| Food | Absorption not affected by food2 | A high-fat meal delays the peak by about 60 minutes and lowers it 29%1 |
| Headache in trials | 11% (10 mg) and 15% (20 mg) vs 5% on placebo2 | 21% (50 mg) and 28% (100 mg) vs 7% on placebo1 |
| Vision changes | Changes in color vision rare (under 0.1%)2 | Abnormal vision, mostly a color tinge: 2% (50 mg), 11% (100 mg) vs 1% on placebo1 |
| Back pain or muscle aches | Back pain 5 to 6% and muscle aches 3 to 4% at 10 to 20 mg2 | Back pain 4% and muscle aches 2 to 4% at 50 to 100 mg1 |
| Nitrates | Contraindicated; wait at least 48 hours before emergency nitrates2 | Contraindicated; the label says the safe gap is unknown1 |
Sildenafil peaks in about an hour and its half-life is about 4 hours, so it is taken shortly before sex.1 Tadalafil peaks in about 2 hours and has a 17.5-hour half-life, so one dose improved erections for up to 36 hours in trials, and a small daily dose reaches a steady level within 5 days.2
The side effects listed on both labels are similar in kind: headache, flushing, indigestion, stuffy nose, back pain and muscle aches.21 The Viagra label reports a color tinge to vision more often at 100 mg (11%).1 The Cialis label notes back pain and muscle aches usually start 12 to 24 hours after a dose and settle within 48 hours.2 The rates come from each drug's own trials, not a head-to-head study.
More detail on tadalafil schedules: Cialis daily vs as needed: how do the two tadalafil schedules differ?. Label numbers: Cialis side effects and the Cialis dosing schedule.