Can I smoke marijuana & take doxy-cycl hycl 100mg cap mutal?

Yes but... There are no known negative interactions between marijuana and doxycycline. If you are taking Doxycycline it is wise to take probiotics including sacchromyces boulardii to reduce resultant imbalances of intestinal flora. I do not know if you are using marijuana recreationally or medicinally but advise using it in moderation.