Newts are generally a safe pet, however there are a few species that are known to carry salmonella, and other species are in fact poisonous, but only if consumed. The brightly colored bellies of a newt are one way to tip a person off to their toxicity. However you are not really at risk unless you plan to do something dumb like lick or eat the newt. In which case, you would notice a horrible burning sensation and most likely experience vomiting.
We know this information thanks to a few drunken idiots who have eaten newts in the past. However this is really no laughing matter, do not be mistaken a newt’s poison is enough to kill you. Within 20 minutes to three hours of ingesting a newt one will typically begin experiencing symptoms of toxic effects. These may appear as hypertension, respiratory paralysis, and nerve unresponsiveness. Less extreme responses may include numbness and/or tingling. Though paralyzed, victims of this toxicity will still remain conscious and is caused by site specific binding of TTX to voltage gated sodium ion channels in the nervous tissues. Any binding extracellular pore opening of this channel inactivates it, blocking sodium ion flow.
Those with extreme doses of TTX can expect cardiac arrhythmia to occur and as a secondary effect of this toxicity, is the inability of brain cells to receive oxygen properly. This results in a lightheaded feeling and overall weakness. The only recommendation for treatment in this case is assisted respiration.
In general, it is safe to keep newts as pet as long as nobody is eating them. The newt will not be able to hurt you or its tank mates unless ingested. It is especially important to not keep newts as pets around children, other animals, or drunken idiots. If you know somebody who ingests a newt then the poison control center should be contacted immediately and/or they should be driven to an emergency room to seek treatment. This is a very serious matter.