A hospital worker in Birmingham locked herself in a room after coming face-to-face with trespassers.
Needles and drug paraphernalia were found in rooms at Birmingham's Heartlands Hospital as staff arrived in the morning.
Blankets were on chairs in waiting areas and it is thought people slept there overnight.
Clinics were cancelled until an area next to the Children's Eye Clinic was made safe by an infection control team.
The orthoptist came upon a woman who was apparently on drugs as she went into work at 07:00 GMT, a source told the BBC.
The source was concerned that children could have found the equipment.
The incident happened on 10 October in the former outpatients clinic 4 at the Bordesley Green site, but that there had been similar activity the previous week.
It was claimed that at the time, there were no locks on doors and no security in the vicinity.
University Hospitals Birmingham said rooms had been decontaminated and added that new security measures would be installed.
The trust said that a small number of appointments unfortunately had to be rescheduled. The area is only in partial use.
Once the trust had been made aware of the situation, senior leaders visited the department to check on the welfare of staff, provide support, and review the security arrangements, it said.
New access controls and additional CCTV will be installed, it added, and all rooms used for illicit drug taking by trespassers had been decontaminated. Any rooms that are not in use have been secured and made inaccessible.
The trust says it has also put on extra security patrols.
"Despite an extensive trawl of footage, we were unable to identify the individuals involved," a spokesperson said.
"We also share all intelligence with our local police liaison officer and work closely with the team to provide drug dog patrols to help keep our patients and colleagues safe."