Best to see a qualified doctor. You may have other problems contributing to this. v1agra may help, but you need to get a prescription, so a check up is in order.
There are many natural herb formulas like Cobra Sex Enhancer
(Cobra rates high on the web, but it is not as powerful as v1agra) that might help a little, is way cheaper and sold at health stores without a prescription. Cobra has side effects.
v1agra seems to be the all time favorite prescription drug, but it has minor side effects and works better on an empty stomach.
(I will state that drinking, smoking and use of drugs have a negative effect on an erection.)