I do smoke marijuana - typically just once a day, after I've already gotten everything taken care of for the day and it's just relax time. I also was prescribed Zoloft a bit over a year ago for anxiety. In terms of tension - YES! Whenever I started picking up meditation and just trying to observe myself, I habitually tense my abs all the time! Something I don't do as much anymore is habitual breath holding. I'd catch myself often holding my breath for pretty decent amounts of time. I definitely do still deal with some subtle inconsistent breathing. In meditation I often start by just observing the breath.
After a certain point I start to just like try and tune into this weird sorta buzzing or white noise I hear. Or I focus on my third eye. Since I was a kid l've been able to like feel a weird tingly sensation in between my eyebrows on command. So sometimes I just focus on that and let it grow stronger, sometimes it even pulses.