Botox injections are a common cosmetic procedure given to reduce facial lines and wrinkles.
The product is made from small, purified doses of botulinum toxin, produced by the bacterium Clostridium botulinum. Larger amounts of the toxin can cause botulism.
According to the UKHSA, the evidence so far suggests clinics involved in the cases have used unlicensed Botox-like products.
In the most recent cases, recorded in East England and the East Midlands, patients had difficulty swallowing, slurred speech and breathing difficulties requiring respiratory support.
Other symptoms of botulism can include droopy eyelids, double vision and weak facial muscles.
Dr Gauri Godbole, of UKHSA, said botulism related to aesthetic procedures was rare but could be serious. She added that symptoms could take up to four weeks to develop and urged anyone who suspected they were suffering to contact the NHS 111 service.
Botox can only be prescribed after a consultation between you and a healthcare professional, such as a doctor or nurse.
While the person signing the prescription doesn't have to give the injection, they should ensure the person injecting is qualified to do so.
Dr Alison Cave, chief safety officer at the Medicines & Healthcare products Regulatory Agency, said: "Buying botulinum toxin in any other circumstances significantly increases the risk of getting a product which is either falsified or not licensed for use in the UK.
"This means that there are no safeguards to ensure products meet the MHRA's standards for quality and safety."
The Joint Council for Cosmetic Practitioners says it receives numerous reports of the "illicit supply and use of unlicensed botulinum toxins".
It suggests those considering Botox injections ask for information about the product, including its brand and intended dose, before accepting a procedure.
People should check these details again with the person carrying out the procedure on the day of their treatment. The prescription for Botox must be in the customer's name.
The UKHSA recommends the following precautions: