A father allegedly watched as his son and his step-daughter drank coffee laced with highly-addictive methamphetamine in Phoenix, Arizona.

Edgar Flores Avila was allegedly smoking methamphetamine in his car, when he placed roughly 0.5 grams of the powerful stimulant into his coffee cup on Thursday (30 November).

Courts documents say that the 33-year-old then watched as his two-year-old son and five-year-old step-daughter drank from the tainted cup which contained the psycho stimulant.

The drug is linked to thousands of overdoses every year throughout the world, but despite the danger Flores Avila did nothing to stop the children drinking the coffee.

Phoenix police say in the documents filed on Thursday (7 December) that the boy immediately turned white and started convulsing in the car as his father watched.

According to ABC News the children used a straw to taste the drink and the girl told police that the concoction was "sour and nasty."

The girl told police, according to 12 News, that the boy turned white and started "moving, moving, moving."

After drinking the coffee both children were rushed to a nearby hospital where they both tested positive for the highly-addictive drug.

According to the court documents, Flores Avila admitted to putting 0.5 grams of meth into the cup and he also tested positive for the drug.

Flores Avila was booked in on child abuse charges and wrote a confession telling police that he "wasn't thinking about what he was doing," reports said.

The drug awareness website Talk to Frank says that the stimulant makes users feel very up, exhilarated, alert and awake, but can also lead to feelings of agitation, paranoia, confusion and aggression.

There's evidence that long-term methamphetamine use can cause brain damage, although this gradually gets better if the user stays off the drug for a long time.

Avila was taken into custody on 7 December.