 Many human medicines should never be given to cats . For example , the painkiller paracetamol ( or acetaminophen , sold as Tylenol and Panadol ) is extremely toxic to cats : even very small doses need immediate treatment and can be fatal . Even aspirin , which is sometimes used to treat arthritis in cats , is much more toxic to them than to humans and must be administered cautiously . Similarly , application of minoxidil ( Rogaine ) to the skin of cats , either accidentally or by well @-@ meaning guardians attempting to counter loss of fur , has sometimes been fatal . Essential oils can be toxic to cats and cases have been reported of serious illnesses caused by tea tree oil , including flea treatments and shampoos containing it . 
