Medicines & Supplements A to Z > Cysteine


Last updated on 16/07/2021.

Overview   |   Dosage   |    Side Effects   |   Precautions  |   Tips  |   Where To Buy  

GENERIC NAME: Cysteine, L-Cysteine



DRUG CLASS: Supplements

SUB CLASS: Amino acid derivatives

USED FOR: Nutritional supplementation

What is Cysteine for?

Cysteine is a non-essential amino acid that is produced in the human body from Methionine. Amino acids are the building blocks that our bodies use to make proteins. The body makes sufficient cysteine to meet its natural needs, however in some individuals who cannot make enough (due to genetic or medical reasons), cysteine is a conditionally essential nutrient and must be obtained from food or supplements.

Cysteine is important in many cell functions and is required for growth and tissue repair. It is abundant in beta-keratin, which is the main protein in nails, skin and hair. It is also found in collagen, which is responsible for skin elasticity. Cysteine is a building block of the natural antioxidant Glutathione, which is essential in many detoxifying processes in the body.

How to use it?

Cysteine is commonly available as tablets or capsules which may be taken by mouth. It is also often found in protein powders, including whey- and plant-based proteins. The daily recommended intake of Methionine with Cysteine together is 14 mg per kg body weight for adults, and 22-27 mg per kg body weight for children aged 2 or above. Do not take more than 7 grams of cysteine per day as it may be toxic to human cells.

Healthy children and adults do not need to consume cysteine from food or supplements because the body produces sufficient amounts from the essential amino acid Methionine to meet daily needs. However, people with cysteine deficiency may take cysteine supplements.

What are the side effects of Cysteine?

Taking cysteine supplements may cause nausea and vomiting, diarrhoea and fatigue.

Who should not take Cysteine?

– People allergic to any component of the formulation
– Patients with diabetes
– Patients with cystinuria, a rare genetic condition of cystine buildup (a substance related to cysteine)

Pharmacist Tips:

– It is always best to consult your doctor or pharmacist before starting any supplement, especially if you already take medication, have health concerns, or are pregnant.
– Most people should obtain adequate amounts of Cysteine from a healthy diet. Most high-protein foods contain cysteine, including fish, meat, dairy products, and legumes.

Common dosing schedule:
Please refer to specific product instructions.

Some common medications Cysteine may interact with:
– Insulin

Please notify your doctor or pharmacist if you are taking the above medication, dosage adjustments might be needed.

Where to buy Cysteine in Hong Kong?

In Hong Kong, Cysteine is an over-the-counter medicine and can be purchased without a prescription from any retail outlet in Hong Kong. This includes grocery stores, convenience stores, medicine shops, and pharmaciesIt can also be obtained from doctors. To find a pharmacy near you, refer to the list of pharmacies (“Authorized Sellers of Poisons”) from the Hong Kong Department of Health.

For more information about drug names and ingredients, you can visit Hong Kong’s Drug Office at

Need more information?

For detailed information about the use of Cysteine (supplement) in children, pregnancy and breastfeeding, and for questions about drug interactions, please check with your pharmacist or doctor.

Disclaimer: our goal is to provide you with the most relevant drug information and common dosage for Hong Kong. This information does not constitute medical advice. For specific treatment recommendations and advice, always discuss with your doctor or pharmacist, and follow the instructions included with your specific medicine.