How to have a poison effect

Hi there

How's everyone?

well, I'll get to the case, you see I'm making a 2D adventure game and I want to have a poison status effect on my character but I can't get a good one and I looked all over in internet and none made the poison damage sensible.

here's a video of what's going on in the game:

https://www.mediafire.com/file/9hg39rxyxp385b3/2018-11-01_16-18-36.flv/file

and here's the code I made (you'll notice more than one code about the poison damage cuz I took it from different sites and I need you help figuring out the mistakes I made):

 public float maxHealth;
 public GameObject deathFX;
 public AudioClip playerHurt;
 
 public float currentHealth;
 PlayerController controlMovement;
 AudioSource playerAS;
 
 public Slider healthBar;
 public Image damageScreen;
 
 bool damaged = false;
 Color damagedColor = new Color(0f, 0f, 0f, .5f);
 float smoothColor = 5f;
 
 /*public bool isPoisoned;
 public bool poisonEffect;
 public int poisonDamage;
 public float poisonTimer;*/
 
 public float HowLongPoisonLasts;
 public float PoisonDamageInterval;
 float nextPoisonDamage;
 public float poisonDamage;
 public bool isPoisoned;
 public bool poisonEffect = true;
 
 // Use this for initialization
 void Start () {
 //isPoisoned = false;
 currentHealth = maxHealth;
 controlMovement = GetComponent<PlayerController>();
 
 healthBar.maxValue = maxHealth;
 healthBar.value = maxHealth;
 damaged = false;
 
 playerAS = GetComponent<AudioSource>();
 }
 
 // Update is called once per frame
 void Update () {
 if (damaged)
 {
 damageScreen.color = damagedColor;
 }
 else
 {
 damageScreen.color = Color.Lerp(damageScreen.color, Color.clear, smoothColor * Time.deltaTime);
 }
 damaged = false;
 
 if (isPoisoned)
 {
 StartCoroutine("PoisonDamage");
 }
 
 if (currentHealth <= 0)
 Die();
 }
 
 public void TakeDamage(float damage)
 {
 if (damage <= 0)
 return;
 
 currentHealth -= damage;
 playerAS.clip = playerHurt;
 playerAS.Play();
 
 isPoisoned = true;
 
 //playerAS.PlayOneShot(playerHurt);
 
 healthBar.value = currentHealth;
 damaged = true;
 
 if (currentHealth <= 0)
 {
 Die();
 }
 }
 
 public void Die()
 {
 Instantiate(deathFX, transform.position, transform.rotation);
 Destroy(gameObject);
 }
 
 IEnumerator PoisonDamage()
 {
 float poisonCounter = 0;
 while (poisonCounter < HowLongPoisonLasts)
 {
 /*isPoisoned = true;
 currentHealth -= poisonDamage * Time.deltaTime;
 healthBar.value = currentHealth;
 yield return new WaitForSeconds(PoisonDamageInterval);
 poisonCounter += PoisonDamageInterval;*/
 
 currentHealth -= poisonDamage;
 healthBar.value = currentHealth;
 nextPoisonDamage = Time.time * PoisonDamageInterval;
 poisonEffect = false;
 yield return new WaitForSeconds(PoisonDamageInterval);
 poisonEffect = true;
 }
 isPoisoned = false;
 }