@retireddoc's comment above explains why I'm taking tadalafil after NS RARP. I'm not a medical professional, but here's a document on UCSF that someone on this blog mentioned in another thread. It has a section on Penile Rehabilitation that includes some information related to your question:
https://urology.ucsf.edu/sites/urology.ucsf.edu/files/uploaded-files/basic-page/managing_erectile_dysfunction_a_patient_guide.pdf
Other than a headache the first couple days after I started taking it, I haven't had any side effects. It does seem to help with increased blood flow to the penis. BTW, I get quite a discount using free Goodrx.com coupons. I haven't tried it yet, but someone on this blog said Mark Cuban's Cost Plus drugs was very inexpensive to buy it, but you have to open an account first before having your prescription sent there. Best wishes.