What can cause frequent yawning throughout the day?

Poor sleep: Due to insomnia. Also consider sleep apnea.... In some individuals their medications can cause fatigue though looking through yours it does not jump at me....

Many: Withdrawal from or tolerance to certain drugs and medicarions are also possible. This is very common with opiates. Stimulants are another common class of drugs which could do this and should be ruled out since you list Phentermine on your profile. Hypothyroidism and Clonazepam as listed in your profile are other potential contributing factors.