Yes, 'tarnish' is correct, Welkins.
About the gerund: 'using' is correct. If you use a modifier for 'using,' though, use 'his,' as in 'his using.'
Actually, you don't need any modifier for 'using' since the performer of 'using' and the subject of the sentence are the same. You could just say, '...by using drugs.' We know who it is who's using drugs.