Hello. Thank you for writing to us at healthcareMagic Doxycycline and lactic acid bacillus capsules is a broad spectrum Oral antibiotic for treatment of active inflammatory acne i.e acne that has red, pustular and painful lesions. It is not a multivitamin and is not useful for treatment of black spots left over by previous acne. Doxycycline is a relatively safe drug for treatment of active acne and the side effects are mainly GI related i.e nausea, diarrhea, abdominal bloating etc. Rare side effect of doxycycline is photosensitivity i.e sensitivity to light and this can manifest as an eruption over sun-exposed skin like face, arms, etc. Weight gain and hair fall is not a side effect of doxycycline. Doxycycline is a prescription drug and should never be taken without supervision of a dermatologist. Dark spots/ black marks from previous acne are better treated with a pigment lightening agent available in a cream form e.g kojic acid, hydroquinone, glycolic acid, azaleic acid etc. These creams are available on prescription of a dermatologist. I suggest you to talk to your doctor for more details and the needful. You may also opt for a few sessions of chemical peeling from your treating dermatologist. Glycolic acid chemical peeling is particularly useful for acne induced hyperpigmentation. Regards