I agree w/ GuitarRox - doubtful this one-time deal would do you much harm. But same as GR, I'd want to know why you took Viagra in the 1st place at age 24 - were you experiencing any symptoms that made you think you were developing ED, or was this just sort of an 'experiment' to see what would happen? First, it's never a good idea to take a Rx drug that wasn't prescribed for you, and 2nd, it's an even worse idea to take a larger dose than prescribed. Normal doses of Viagra are 50 mg & 100 mg, altho I've heard that in difficult cases of ED, some have needed 200 mg to get the expected results, presumably with the prescribing Dr's blessing. It would be unusual for a guy of 24 to need ED meds, but sometimes other physical conditions, treatments, etc., could cause problems. But in the future, if you ever need help with ED, please see you regular Dr first, and possibly a Urologist as well - ED can also be an early sign of cardiovascular disease, so you should be thoroughly checked out before getting a prescription. The Dr needs to know why you're having this issue & whether a Rx for Viagra, Cialis or Levitra would be warranted...
What do you mean you once took 300 mg of Viagra? Like all at once? That's too much at once. Were these your pills or someone else's because no doctor would prescribe that amount at one time. Or are you talking about multiple times you took it to total 300 mg? Clarify please.
But no, you shouldn't have lasting erection issues. Why did you take it in the first place?