I take Gabapentin 900mg per day, Tizanidine 4mg as needed, Levothyroxine 75mcg, Nitrofurantoin temp) and Adipex-P. Interactions?

No interactions. there is no interaction between these meds. Please always check with your physician if you have health questions or concerns.
Unusual med combo. First of all, the question of adding Gabapentin to any medication to any med combination is that this antiseizure medication is eliminated from your blood through an glucoronic acid conjugation and it is washed out in urine. On the other hand the combination of fentermine to lose weight and a muscle relaxant plus a thyroid hormone replacement medication is not quite frequent.