The Department of Health on Monday warned the public not to take an oral product called Hemohim, saying it can cause liver damage.
The department said the Hospital Authority had notified it on Friday of suspected poisoning cases, involving acute liver injury, in four women, aged between 42 and 72.
All had reported tea-coloured urine, jaundice, and vomiting between April and September. They were admitted to hospital and have since been discharged.
The women said they had taken Hemohim, thinking it would improve their health, with consumption ranging between about two weeks and six months.
The department collected two samples from the patients and the Government Laboratory on Monday confirmed the presence of methoxsalen in all samples collected.
A Department of Health spokesman said methoxsalen was considered a probable cause of liver injury. Other common side effects include nausea, headache, dizziness, fatigue, depression and skin reaction to sunlight.
Methoxsalen - derived from the plant Ammi Majus, known commonly as Bishop's Weed or Queen Anne's lace - is a substance listed under the pharmacy and poison regulations. It can be used to treat diseases such as psoriasis and vitiligo.
The department said it was investigating the source of the product and would take enforcement action when necessary.