Coming from a pharmacist, always check with a doctor first before you take any pills. They will assess if you are suitable, if you are fit, don't have any health conditions that might pre-dispose you to certain side effects.
After that, make sure you get it from a reputable source.
If you are scared of priapism, a common remedy is pseudoephedrine which acts as a counter-balance to the viagra. If that doesn't work people will get the penis drained.
Priapism is not a common side effect so I would not worry too much - just be aware.
Do not take any of this medical information seriously without consulting your doctor though!