Yes, the oldstyle mothballs using Naphthalene or Paradichlorobenzene are poisonous. There are newer ones that use different chemicals. I don't know their effectiveness though.
The oldstyle mothballs are useful to keep critters out of your garden, both crawling insects and cats. From a safety standpoint I avoid using them indoors.
Try this site for more info on mothballs. They have an overview and many links for further information. They are a partnership of Oregon State University and the U.S. Environmental Protection Agency.
http://npic.orst.edu/ingred/ptype/mothball/health.html
For indoor use and a little more effort involved, Boric Acid powder is a very good general insecticide and fungicide. The advantages are it pretty much stays in place so it doesn't foul the air, and continues to work almost forever. Not recommended for use in food storage cupboards. In the USA it is sold in Garden Supply stores to fight insects. Many years ago it was also available in Drug stores.
The extra work is it is a contact poison, so you have to put it where the roaches are. Pay attention to where they mostly appear and get the powder into any cracks in the vicinity. It may take some detective work (underneath a countertop for instance, or inside a recently acquired padded chair). If you can't pinpoint a specific area, there are roach traps available (at least in the USA) that you scatter around. The traps that catch the most roaches are closest to their hideout(s).
A few residents of the building I'm in have had roach investations and have used the management-supplied exterminator service. The exterminator sprays some smelly liquid around and tells them to stay out for at least four hours. Then repeats the next 2 or 3 months.
Other residents have used Boric Acid with good results once the hiding places are found. (One area was the crack at the wall and floor behind the refrigerator. Another was between a Granite countertop and the uneven cabinet it was on.) The downside is some roaches will crawl out into the open to die.
Some Boric Acid comments and cautions: Boric Acid, at least in the past, was commonly the active ingredient in Eye Wash products, as a saturated solution in water it is effective in controlling fungus infections such as athlete's foot and fungus infection on other body parts. It is quite poisonous if ingested, (when major powder application is needed, I usually use a mask to limit inhalation.) Do not get it on broken skin or in a wound. Wash hands after use.
Cheers,
Tom