1. Rifampin (Rifampicin)
Orange-red discoloration of urine, sweat, tears, and saliva.
Harmless and temporary — a sign the medicine is working.
2. Metformin
Nausea, diarrhea, or metallic taste, especially when starting.
Usually improves within a few days. Take with meals.
3. Statins (e.g. Atorvastatin, Simvastatin)
Mild muscle aches or fatigue.
Common reaction. Report only if severe, persistent or with dark urine.
4. Iron Supplements
Dark or black stools. Usually normal and harmless.
5. Amoxicillin & certt antibiotics:
Loose stools or mild diarrhea.
Due to temporary changes in gut bacteria.
6. Opioids (e.g. Pentazocine, Codeine, Oxycodone, Morphine)
Constipation and mild nausea when starting.
Common because they slow gut movement. Laxatives are often recommended.

Bottom line
These are expected side effects, not signs of a problem. Always consult your pharmacist or doctor if symptoms are severe or concerning.