She appeared primarily in Seasons 1-5, 7 & 8

Does anyone know if she was in any of the episodes in seasons 6 & 9?
If so, which ones?

As for her downward spiral, can anyone pinpoint which episodes (in seasons 4 & 5) any physical changes or a reduction in her lines started occurring?

She looks slightly different in the Season 4 and Season 5 credits Josh posted on YouTube.

Also, how did they finally write her off the show the second (and final) time?

She just ran off and left her baby and husband behind. Not a very good way for the character to end.

She did appear in a few season 9 episodes.

Shakedown - Season 9
The Dentist - Season 9

I am currenly watching season 9 again so I will check to see what other episodes she was in because IMDB is wrong on a few episodes

From another source besides IMDB,lists the additional 2 more episodes from Season 9,which are:

Take My Ex
The Bedtime Story

The source also lists Season 9 Ep 23 One day at a time Reunion 2/22/05

So she was in the first 4 episodes of Season 9....and I'm guessing no episodes in Season 6.

Now what about her physical changes due to drug use....what episodes do you see the change beginning, or at it's peak?

Funny how she looks more like Ann's neighbor than her daughter later on during the series

From another source besides IMDB,lists the additional 2 more episodes from Season 9,which are:
Take My Ex
The Bedtime Story
The source also lists Season 9 Ep 23 One day at a time Reunion 2/22/05

She did not appear in Take My Ex. I have the unedited episode and she is not in it.

Aha! I think she was supposed to appear in that episode but I think that is when she left the show during the rehearsal for THAT episode. I believe when she was there for rehearsal, that is when she was asked to take a drug test. She walks off and never gets around to taping in that episode. Bam! Got it!

So she was in the first 4 episodes of Season 9....and I'm guessing no episodes in Season 6.

Now what about her physical changes due to drug use....what episodes do you see the change beginning, or at it's peak?

Later in season four: it is noticeable and in season five was just obvious.

Late season eight, she was losing the weight that she gained and by the start of the ninth and final season, she was a walking dead zombie skeleton. She could not be filmed closeup because of her appearance, according to Mack, herself. They tried giving her different looks, makeovers and hairstyles to try and hide flaws but they were still noticeable. Posting the cast pic again from the last season and if you notice, Mack is in the back.

Which episodes in Seasons 5, 7 & 8?

Sorry Josh....I couldn't find it on Google lol