Doedrant contains alcohol,so can we offer namaz after using it?

No.
Whenever you hear someone say if you do this or that you cannot make namaz, it`s from Ibliis 99.9% of the cases.
Haram to drink is not Haram to make Namaz having the scent of it on your skin. Besides, alcohol dries up from your skin in about 10 seconds after using it externally.
Only it`s polite for the angels to prefer parfume without alcohol, because the smell of alcohol may not be pleasing to them, because they are harmed by what harms Banuu Adam.
But Allaah knows best, because even alcohol is sometimes not hurting children of Adam - and Allaah said that in it is also benefit for people, but the evil is more. What if you are in a car accident, will it be fardh upon you to forbid the doctor and nurses from cleaning your wounds by sterilizing them with alcohol, because otherwise you can`t make salaah? If it was, then it would be better wAllaahi to die of infection than to have them treat you. But the Religion of Allaah is not ordering this. These are people who follow their whims and desires who invent evil things and lie about Allaah.
Keep always your namaz, and turn away from these liars. They only try to excuse themselves from not praying, because Allaah has lead them astray due to the evil in their hearts, and Allaah doesn`t want to see them standing to Him.