Unfortunately the side effects are not standard in everyone, sometimes they do lesson or disappear as you continue to take the medication, in others they never get worse but do not improve either, the 3 week reference i beleive is more in reference to se if medication is going to help with your depression, it is always advised to take a anitdepresant for at least 3 weeks to give it a chance.
back to your taking of the drug, is your erection harder and painful with this medication, or just normal and you just can cum?