diff --git a/src/components/settings.tsx b/src/components/settings.tsx
index a989263..c37e6b7 100644
--- a/src/components/settings.tsx
+++ b/src/components/settings.tsx
@@ -193,7 +193,7 @@ const Settings = ({
           <Label className="font-medium">{t('halfLife')}</Label>
           <FormNumericInput
             value={pkParams.damph.halfLife}
-            onChange={val => onUpdatePkParams('damph', { halfLife: val })}
+            onChange={val => onUpdatePkParams('damph', { ...pkParams.damph, halfLife: val })}
             increment={0.5}
             min={0.1}
             unit="h"
@@ -209,7 +209,7 @@ const Settings = ({
           <Label className="font-medium">{t('conversionHalfLife')}</Label>
           <FormNumericInput
             value={pkParams.ldx.halfLife}
-            onChange={val => onUpdatePkParams('ldx', { halfLife: val })}
+            onChange={val => onUpdatePkParams('ldx', { ...pkParams.ldx, halfLife: val })}
             increment={0.1}
             min={0.1}
             unit="h"
@@ -222,7 +222,7 @@ const Settings = ({
           <Label className="font-medium">{t('absorptionRate')}</Label>
           <FormNumericInput
             value={pkParams.ldx.absorptionRate}
-            onChange={val => onUpdatePkParams('ldx', { absorptionRate: val })}
+            onChange={val => onUpdatePkParams('ldx', { ...pkParams.ldx, absorptionRate: val })}
             increment={0.1}
             min={0.1}
             unit={t('faster')}
diff --git a/src/locales/de.ts b/src/locales/de.ts
index 87bb83f..f01499d 100644
--- a/src/locales/de.ts
+++ b/src/locales/de.ts
@@ -100,6 +100,11 @@ export const de = {
   errorTimeRequired: "Bitte gib eine gültige Zeitangabe ein.",
   warningDuplicateTime: "Mehrere Dosen zur gleichen Zeit.",
   warningZeroDose: "Nulldosis hat keine Auswirkung auf die Simulation.",
+  halfLifeRequired: "Halbwertszeit ist erforderlich.",
+  conversionHalfLifeRequired: "Umwandlungs-Halbwertszeit ist erforderlich.",
+  absorptionRateRequired: "Absorptionsrate ist erforderlich.",
+  therapeuticRangeMinRequired: "Minimaler therapeutischer Bereich ist erforderlich.",
+  therapeuticRangeMaxRequired: "Maximaler therapeutischer Bereich ist erforderlich.",
 
   // Day-based schedule
   regularPlan: "Regulärer Plan",
diff --git a/src/locales/en.ts b/src/locales/en.ts
index 4642e10..235ac3e 100644
--- a/src/locales/en.ts
+++ b/src/locales/en.ts
@@ -99,6 +99,11 @@ export const en = {
   errorTimeRequired: "Please enter a valid time.",
   warningDuplicateTime: "Multiple doses at same time.",
   warningZeroDose: "Zero dose has no effect on simulation.",
+  halfLifeRequired: "Half-life is required.",
+  conversionHalfLifeRequired: "Conversion half-life is required.",
+  absorptionRateRequired: "Absorption rate is required.",
+  therapeuticRangeMinRequired: "Minimum therapeutic range is required.",
+  therapeuticRangeMaxRequired: "Maximum therapeutic range is required.",
 
   // Time picker
   timePickerHour: "Hour",
diff --git a/src/utils/pharmacokinetics.ts b/src/utils/pharmacokinetics.ts
index 7ee87e0..688da18 100644
--- a/src/utils/pharmacokinetics.ts
+++ b/src/utils/pharmacokinetics.ts
@@ -25,9 +25,20 @@ export const calculateSingleDoseConcentration = (
   const numDose = parseFloat(dose) || 0;
   if (timeSinceDoseHours < 0 || numDose <= 0) return { ldx: 0, damph: 0 };
 
-  const ka_ldx = Math.log(2) / (parseFloat(pkParams.ldx.absorptionRate) || 1);
-  const k_conv = Math.log(2) / (parseFloat(pkParams.ldx.halfLife) || 1);
-  const ke_damph = Math.log(2) / (parseFloat(pkParams.damph.halfLife) || 1);
+  const absorptionRate = parseFloat(pkParams.ldx.absorptionRate);
+  const conversionHalfLife = parseFloat(pkParams.ldx.halfLife);
+  const damphHalfLife = parseFloat(pkParams.damph.halfLife);
+
+  // Validate parameters to avoid division by zero or invalid calculations
+  if (isNaN(absorptionRate) || absorptionRate <= 0 ||
+      isNaN(conversionHalfLife) || conversionHalfLife <= 0 ||
+      isNaN(damphHalfLife) || damphHalfLife <= 0) {
+    return { ldx: 0, damph: 0 };
+  }
+
+  const ka_ldx = Math.log(2) / absorptionRate;
+  const k_conv = Math.log(2) / conversionHalfLife;
+  const ke_damph = Math.log(2) / damphHalfLife;
 
   let ldxConcentration = 0;
   if (Math.abs(ka_ldx - k_conv) > 0.0001) {