This is a simply a collection of information I've gathered through research and partial experience. I am not really an expert, so if somethings off or wrong, input is greatly appreciated!
The basic definition of a thought form is something given shape and purpose by a person. They are not beings that should be taken lightly. Do not hesitate to dismiss.
The physical effects a thoughtform can have on a person can be varied in some cases. However, their most basic effect is an energy drain. They eat your 'energy' so they can live. Constant regulation of this is needed. A person must set limits. If there are not any established, the energy drain can be taken advantage of and the 'host' can be chronically exhausted and that can have other bad effects. This can be the same case with creating multiple thoughtforms. Once again, set rules. If the rules cannot be set/ thoughtform has fulfilled its intended purpose, it would be best to dismiss it.
For Thoughtforms, their 'base' is very important. One should not be made sloppily because the repercussions can be immense. If one is made with blind anger and sent on some vengeance, expect bad results. They need time and specification. If one cannot be handled, dismiss or reinforce the imposed 'rules' somehow. In any situation that is too large and cannot be handled, dismiss it as quickly as possible.
A long term Thoughtform is a huge undertaking. For those I know, with the right handling they've gained an indispensable being. However, there are many negatives to making a long term Thoughtform. The rules need to be constantly enforced and tweaked with some. The Thoughtforms can be akin to developing children and will more often then not cause their own versions of tantrums. It takes perseverance to raise them. In addition, the longer 'lives' they live, the more complicated Thoughtforms can become. Dismissal can be a trial in itself.