Penegra 100 mg (sildenafil citrate 100 mg) is a marketed medicine in India and is available through pharmacies, though availability can vary by location and pharmacy. Because 100 mg is the maximum usual single dose of sildenafil, it shouldn't be taken casually if you've never used sildenafil before. A lower dose such as 25–50 mg may be more appropriate for some people. If you want, I can also compare Penegra 100 mg vs Suhagra 25 mg vs Tadalis SX 20 mg for duration, strength, and side effects. 

Penegra 100 mg contains sildenafil citrate 100 mg and is Penegra 100 mg to treat erectile dysfunction (ED). Onset: usually about 30–60 minutes. Duration: typically around 4 hours, sometimes longer. Sexual stimulation is still required for an erection. 100 mg is the maximum recommended single sildenafil dose for most adults; many people respond well to 25–50 mg. Common effects: headache, flushing, indigestion, nasal congestion, dizziness, and visual changes. Never combine it with nitrate medicines (e.g., nitroglycerin/isosorbide) because blood pressure can fall dangerously. Don't combine it with other ED medicines such as tadalafil (Tadalis) unless a doctor specifically advises it. If you're deciding between Penegra 100 mg and Tadalis SX 20 mg, tadalafil generally lasts much longer (up to ~36 hours), while sildenafil generally has a shorter window.