Charlie Sheen and Brooke Mueller's custody agreement does not require them to undergo drug testing.

The former couple -- who have two-year-old twin boys Bob and Max together -- reached a joint custody deal in June and while details of the agreement were not filed in court, in order to keep the terms completely confidential, gossip website TMZ claims part of the agreement is that neither Charlie nor Brooke have to undergo drug tests.

While the couple -- who have both struggled with substance abuse problems -- share legal and physical custody of their children, Brooke is the primary caregiver, meaning she gets to say how they are looked after the majority of the time.

Brooke was previously juggling her parental responsibilities with rehab, completing a 45 day outpatient programme which allowed her to spend eight hours a day with Bob and Max on June 1.

At the time, friends were very positive, with one pal saying: "Brooke is just taking it a day at a time. She has been down this road before and she knows what she has to do. Brooke needs to continue to play an active role in her sobriety."

However, according to TMZ, Brooke has since been photographed holding what appeared to be a crack pipe and photographed in Hawaii, walking up to a parked car in a "seedy" area with cash in hand.

Meanwhile, little has been heard of Charlie since he completed his "Violent Torpedo of Truth/Defeat is not an Option" stage tour at the beginning of the summer.

In the wake of being fired from sitcom "Two and a Half Men" in March he renamed his house the "Sober Valley Lodge" and took drug tests to prove he was clean, but it is unknown if this is still the case.

It is thought the Los Angeles County Department of Children and Family Services has not visited either Charlie or Brooke for more than six months and no judge has ordered a hearing to determine if they are unfit parents.

Brooke and Charlie married in 2008, but finally split in April 2010 after their separate problems with drugs and lifestyle choices proved too much for their marriage, with their divorce was finalized in May this year.